How does Google Instant work?

UPDATE: Google have just published a blog article called Google Instant, behind the scenes. It's an interesting read, and obviously related to this question. You can read how they tackled the extra load (5-7X according to the article) on the server-side, for example. The answer below examines what happens on the client-side:

Examining with Firebug, Google is doing an Ajax GET request on every keypress:

I guess it's working the same way as the auto completion. However this time, it also returns the search results of the partially complete search phrase in JSON format.

Examining one of the JSON responses while typing "Stack Overflow":

We can see that the JSON response contains the content to construct the search results as we type.

The formatted JSON responses look something like this:

{
    e: "j9iHTLXlLNmXOJLQ3cMO",
    c: 1,
    u: "http://www.google.com/search?hl\x3den\x26expIds\x3d17259,24472,24923,25260,25901,25907,26095,26446,26530\x26sugexp\x3dldymls\x26xhr\x3dt\x26q\x3dStack%20Overflow\x26cp\x3d6\x26pf\x3dp\x26sclient\x3dpsy\x26aq\x3df\x26aqi\x3dg4g-o1\x26aql\x3d\x26oq\x3dStack+\x26gs_rfai\x3d\x26pbx\x3d1\x26fp\x3df97fdf10596ae095\x26tch\x3d1\x26ech\x3d1\x26psi\x3dj9iHTO3xBo2CONvDzaEO12839712156911",
    d: "\x3clink rel\x3dprefetch href\x3d\x22http://stackoverflow.com/\x22\x3e\x3cscript\x3eje.pa(_loc, \x27rso\x27, \x27\\x3c!--m--\\x3e\\x3clink rel\\x3dprefetch href\\x3d\\x22http://stackoverflow.com/\\x22\\x3e\\x3cli class\\x3dg\\x3e\\x3ch3 class\\x3d\\x22r\\x22\\x3e\\x3ca href\\x3d\\x22http://stackoverflow.com/\\x22 class\\x3dl onmousedown\\x3d\\x22return rwt(this,\\x27\\x27,\\x27\\x27,\\x27\\x27,\\x271\\x27,\\x27AFQjCNERidL9Hb6OvGW93_Y6MRj3aTdMVA\\x27,\\x27\\x27,\\x270CBYQFjAA\\x27)\\x22\\x3e\\x3cem\\x3eStack Overflow\\x3c/em\\x3e\\x3c/a\\x3e\\x3c/h3\\x3e\\x3cdiv class\\x3d\\x22s\\x22\\x3eA language-independent collaboratively edited question and answer site for programmers.\\x3cbr\\x3e\\x3cspan class\\x3df\\x3e\\x3ccite\\x3e\\x3cb\\x3estackoverflow\\x3c/b\\x3e.com/\\x3c/cite\\x3e - \\x3cspan class\\x3dgl\\x3e\\x3ca href\\x3d\\x22http://webcache.googleusercontent.com/search?q\\x3dcache:U1GC2GYOToIJ:stackoverflow.com/+Stack+Overflow\\x26amp;cd\\x3d1\\x26amp;hl\\x3den\\x26amp;ct\\x3dclnk\\x22 onmousedown\\x3d\\x22return rwt(this,\\x27\\x27,\\x27\\x27,\\x27\\x27,\\x271\\x27,\\x27AFQjCNFfKMag7Tq8CMbbfu8Gcj_GjukTbA\\x27,\\x27\\x27,\\x270CBgQIDAA\\x27)\\x22\\x3eCached\\x3c/a\\x3e - \\x3ca href\\x3d\\x22/search?hl\\x3den\\x26amp;q\\x3drelated:stackoverflow.com/+Stack+Overflow\\x26amp;tbo\\x3d1\\x26amp;sa\\x3dX\\x26amp;ei\\x3dj9iHTLXlLNmXOJLQ3cMO\\x26amp;sqi\\x3d2\\x26amp;ved\\x3d0CBkQHzAA\\x22\\x3eSimilar\\x3c/a\\x3e\\x3c/span\\x3e\\x3c/span\\x3e\\x3cbr\\x3e\\x3ctable class\\x3dslk style\\x3d\\x22border-collapse:collapse;margin-top:4px\\x22\\x3e\\x3ctr\\x3e\\x3ctd style\\x3d\\x22padding-left:14px;vertical-align:top\\x22\\x3e\\x3cdiv class\\x3dsld\\x3e\\x3ca class\\x3dsla href\\x3d\\x22http://stackoverflow.com/questions\\x22 onmousedown\\x3d\\x22return rwt(this,\\x27\\x27,\\x27\\x27,\\x27\\x27,\\x271\\x27,\\x27AFQjCNHmP78gEctJKvBrydP2c52F_FEjvA\\x27,\\x27\\x27,\\x270CBoQqwMoADAA\\x27)\\x22\\x3eQuestions\\x3c/a\\x3e\\x3c/div\\x3e\\x3cdiv class\\x3dsld\\x3e\\x3ca class\\x3dsla href\\x3d\\x22http://stackoverflow.com/questions/ask\\x22 onmousedown\\x3d\\x22return rwt(this,\\x27\\x27,\\x27\\x27,\\x27\\x27,\\x271\\x27,\\x27AFQjCNGZF-qwWVTZOWPlr4vgSA7qB64LLQ\\x27,\\x27\\x27,\\x270CBsQqwMoATAA\\x27)\\x22\\x3eAsk Question\\x3c/a\\x3e\\x3c/div\\x3e\\x3cdiv class\\x3dsld\\x3e\\x3ca class\\x3dsla href\\x3d\\x22http://stackoverflow.com/users/135152/omg-ponies\\x22 onmousedown\\x3d\\x22return rwt(this,\\x27\\x27,\\x27\\x27,\\x27\\x27,\\x271\\x27,\\x27AFQjCNE9zo6Qi_AM1bjmPGeMGfbnPi3niA\\x27,\\x27\\x27,\\x270CBwQqwMoAjAA\\x27)\\x22\\x3eOMG Ponies\\x3c/a\\x3e\\x3c/div\\x3e\\x3cdiv class\\x3dsld\\x3e\\x3ca class\\x3dsla href\\x3d\\x22http://careers.stackoverflow.com/\\x22 onmousedown\\x3d\\x22return rwt(this,\\x27\\x27,\\x27\\x27,\\x27\\x27,\\x271\\x27,\\x27AFQjCNEaqlBrfDcc1gdPZ6dgthff0s5WmA\\x27,\\x27\\x27,\\x270CB0QqwMoAzAA\\x27)\\x22\\x3eCareers\\x3c/a\\x3e\\x3c/div\\x3e\\x3ctd style\\x3d\\x22padding-left:14px;vertical-align:top\\x22\\x3e\\x3cdiv class\\x3dsld\\x3e\\x3ca class\\x3dsla href\\x3d\\x22http://stackoverflow.com/about\\x22 onmousedown\\x3d\\x22return rwt(this,\\x27\\x27,\\x27\\x27,\\x27\\x27,\\x271\\x27,\\x27AFQjCNEqgPttrXj3r4o3TZHX5WaWvFe1HQ\\x27,\\x27\\x27,\\x270CB4QqwMoBDAA\\x27)\\x22\\x3eAbout\\x3c/a\\x3e\\x3c/div\\x3e\\x3cdiv class\\x3dsld\\x3e\\x3ca class\\x3dsla href\\x3d\\x22http://stackoverflow.com/faq\\x22 onmousedown\\x3d\\x22return rwt(this,\\x27\\x27,\\x27\\x27,\\x27\\x27,\\x271\\x27,\\x27AFQjCNF3X3eRc0RsxYynXZhhbyYkuKWZ5g\\x27,\\x27\\x27,\\x270CB8QqwMoBTAA\\x27)\\x22\\x3eThe FAQ\\x3c/a\\x3e\\x3c/div\\x3e\\x3cdiv class\\x3dsld\\x3e\\x3ca class\\x3dsla href\\x3d\\x22http://blog.stackoverflow.com/\\x22 onmousedown\\x3d\\x22return rwt(this,\\x27\\x27,\\x27\\x27,\\x27\\x27,\\x271\\x27,\\x27AFQjCNG7KphjK6RuC5cj-6U5jeuvipt5dg\\x27,\\x27\\x27,\\x270CCAQqwMoBjAA\\x27)\\x22\\x3eBlog\\x3c/a\\x3e\\x3c/div\\x3e\\x3cdiv class\\x3dsld\\x3e\\x3ca class\\x3dsla href\\x3d\\x22http://stackoverflow.com/users\\x22 onmousedown\\x3d\\x22return rwt(this,\\x27\\x27,\\x27\\x27,\\x27\\x27,\\x271\\x27,\\x27AFQjCNFfN_wcGm4HE5XpDxvcH4bIrkv2dw\\x27,\\x27\\x27,\\x270CCEQqwMoBzAA\\x27)\\x22\\x3eUsers\\x3c/a\\x3e\\x3c/div\\x3e\\x3ctr\\x3e\\x3ctd colspan\\x3d2 style\\x3d\\x22padding-left:14px;vertical-align:top\\x22\\x3e\\x3cdiv style\\x3d\\x22padding-top:6px\\x22\\x3e\\x3ca class\\x3dfl href\\x3d\\x22/search?hl\\x3den\\x26amp;q\\x3d+site:stackoverflow.com+Stack+Overflow\\x26amp;sa\\x3dX\\x26amp;ei\\x3dj9iHTLXlLNmXOJLQ3cMO\\x26amp;sqi\\x3d2\\x26amp;ved\\x3d0CCIQrAM\\x22\\x3eMore results from stackoverflow.com\\x26nbsp;\\x26raquo;\\x3c/a\\x3e\\x3c/div\\x3e\\x3c/table\\x3e\\x3c/div\\x3e\\x3c!--n--\\x3e\\x3c!--m--\\x3e\\x3cli class\\x3dg\\x3e\\x3ch3 class\\x3d\\x22r\\x22\\x3e\\x3ca href\\x3d\\x22http://stackoverflow.com/questions\\x22 class\\x3dl onmousedown\\x3d\\x22return rwt(this,\\x27\\x27,\\x27\\x27,\\x27\\x27,\\x272\\x27,\\x27AFQjCNHmP78gEctJKvBrydP2c52F_FEjvA\\x27,\\x27\\x27,\\x270CCUQFjAB\\x27)\\x22\\x3eHottest Questions - \\x3cem\\x3eStack Overflow\\x3c/em\\x3e\\x3c/a\\x3e\\x3c/h3\\x3e\\x3cdiv class\\x3d\\x22s\\x22\\x3eHello \\x3cem\\x3eStack Overflow\\x3c/em\\x3e! I\\x26#39;m working with someone else\\x26#39;s PHP function that works fine as long as I pass it at least three arguments. If I pass it two argument, \\x3cb\\x3e...\\x3c/b\\x3e\\x3cbr\\x3e\\x3cspan class\\x3df\\x3e\\x3ccite\\x3e\\x3cb\\x3estackoverflow\\x3c/b\\x3e.com/questions\\x3c/cite\\x3e - \\x3cspan class\\x3dgl\\x3e\\x3ca href\\x3d\\x22http://webcache.googleusercontent.com/search?q\\x3dcache:6S_0sErDKfQJ:stackoverflow.com/questions+Stack+Overflow\\x26amp;cd\\x3d2\\x26amp;hl\\x3den\\x26amp;ct\\x3dclnk\\x22 onmousedown\\x3d\\x22return rwt(this,\\x27\\x27,\\x27\\x27,\\x27\\x27,\\x272\\x27,\\x27AFQjCNH7WHzefYlnS05ln4j6rzfE3byDKg\\x27,\\x27\\x27,\\x270CCcQIDAB\\x27)\\x22\\x3eCached\\x3c/a\\x3e - \\x3ca href\\x3d\\x22/search?hl\\x3den\\x26amp;q\\x3drelated:stackoverflow.com/questions+Stack+Overflow\\x26amp;tbo\\x3d1\\x26amp;sa\\x3dX\\x26amp;ei\\x3dj9iHTLXlLNmXOJLQ3cMO\\x26amp;sqi\\x3d2\\x26amp;ved\\x3d0CCgQHzAB\\x22\\x3eSimilar\\x3c/a\\x3e\\x3c/span\\x3e\\x3c/span\\x3e\\x3c/div\\x3e\\x3c!--n--\\x3e\\x3c!--m--\\x3e\\x3cli class\\x3dg\\x3e\\x3ch3 class\\x3d\\x22r\\x22\\x3e\\x3ca href\\x3d\\x22http://en.wikipedia.org/wiki/Stack_overflow\\x22 class\\x3dl onmousedown\\x3d\\x22return rwt(this,\\x27\\x27,\\x27\\x27,\\x27\\x27,\\x273\\x27,\\x27AFQjCNEAxaeWmWtD7cBcmZ5QBMsTRNbnCw\\x27,\\x27\\x27,\\x270CCkQFjAC\\x27)\\x22\\x3e\\x3cem\\x3eStack overflow\\x3c/em\\x3e - Wikipedia, the free encyclopedia\\x3c/a\\x3e\\x3c/h3\\x3e\\x3cdiv class\\x3d\\x22s\\x22\\x3eIn software, a \\x3cem\\x3estack overflow\\x3c/em\\x3e occurs when too much memory is used on the call stack. The call stack contains a limited amount of memory,  often determined at \\x3cb\\x3e...\\x3c/b\\x3e\\x3cbr\\x3e\\x3cspan class\\x3df\\x3e\\x3ccite\\x3een.wikipedia.org/wiki/\\x3cb\\x3eStack\\x3c/b\\x3e_\\x3cb\\x3eoverflow\\x3c/b\\x3e\\x3c/cite\\x3e - \\x3cspan class\\x3dgl\\x3e\\x3ca href\\x3d\\x22http://webcache.googleusercontent.com/search?q\\x3dcache:mWu8b0BQAmwJ:en.wikipedia.org/wiki/Stack_overflow+Stack+Overflow\\x26amp;cd\\x3d3\\x26amp;hl\\x3den\\x26amp;ct\\x3dclnk\\x22 onmousedown\\x3d\\x22return rwt(this,\\x27\\x27,\\x27\\x27,\\x27\\x27,\\x273\\x27,\\x27AFQjCNFG_5ndK-KmWJy6s3pOsi8lsxqEZg\\x27,\\x27\\x27,\\x270CCsQIDAC\\x27)\\x22\\x3eCached\\x3c/a\\x3e - \\x3ca href\\x3d\\x22/search?hl\\x3den\\x26amp;q\\x3drelated:en.wikipedia.org/wiki/Stack_overflow+Stack+Overflow\\x26amp;tbo\\x3d1\\x26amp;sa\\x3dX\\x26amp;ei\\x3dj9iHTLXlLNmXOJLQ3cMO\\x26amp;sqi\\x3d2\\x26amp;ved\\x3d0CCwQHzAC\\x22\\x3eSimilar\\x3c/a\\x3e\\x3c/span\\x3e\\x3c/span\\x3e\\x3c/div\\x3e\\x3c!--n--\\x3e\\x3c!--m--\\x3e\\x3cli class\\x3dg\\x3e\\x3ch3 class\\x3d\\x22r\\x22\\x3e\\x3ca href\\x3d\\x22http://blog.stackoverflow.com/\\x22 class\\x3dl onmousedown\\x3d\\x22return rwt(this,\\x27\\x27,\\x27\\x27,\\x27\\x27,\\x274\\x27,\\x27AFQjCNG7KphjK6RuC5cj-6U5jeuvipt5dg\\x27,\\x27\\x27,\\x270CC0QFjAD\\x27)\\x22\\x3eBlog – \\x3cem\\x3eStack Overflow\\x3c/em\\x3e\\x3c/a\\x3e\\x3c/h3\\x3e\\x3cdiv class\\x3d\\x22s\\x22\\x3e6 Sep 2010 \\x3cb\\x3e...\\x3c/b\\x3e The latest version of the \\x3cem\\x3eStack Overflow\\x3c/em\\x3e Trilogy Creative Commons Data Dump is now available. This reflects all public  data in … \\x3cb\\x3e...\\x3c/b\\x3e\\x3cbr\\x3e\\x3cspan class\\x3df\\x3e\\x3ccite\\x3eblog.\\x3cb\\x3estackoverflow\\x3c/b\\x3e.com/\\x3c/cite\\x3e - \\x3cspan class\\x3dgl\\x3e\\x3ca href\\x3d\\x22http://webcache.googleusercontent.com/search?q\\x3dcache:iqtvg9Ge1c0J:blog.stackoverflow.com/+Stack+Overflow\\x26amp;cd\\x3d4\\x26amp;hl\\x3den\\x26amp;ct\\x3dclnk\\x22 onmousedown\\x3d\\x22return rwt(this,\\x27\\x27,\\x27\\x27,\\x27\\x27,\\x274\\x27,\\x27AFQjCNFX2P2-RTCs_GaR6NgSw30p007UEA\\x27,\\x27\\x27,\\x270CC8QIDAD\\x27)\\x22\\x3eCached\\x3c/a\\x3e - \\x3ca href\\x3d\\x22/search?hl\\x3den\\x26amp;q\\x3drelated:blog.stackoverflow.com/+Stack+Overflow\\x26amp;tbo\\x3d1\\x26amp;sa\\x3dX\\x26amp;ei\\x3dj9iHTLXlLNmXOJLQ3cMO\\x26amp;sqi\\x3d2\\x26amp;ved\\x3d0CDAQHzAD\\x22\\x3eSimilar\\x3c/a\\x3e\\x3c/span\\x3e\\x3c/span\\x3e\\x3c/div\\x3e\\x3c!--n--\\x3e\x27,_ss);\x3c/script\x3e"
}/*""*/{
    e: "j9iHTLXlLNmXOJLQ3cMO",
    c: 1,
    u: "http://www.google.com/search?hl\x3den\x26expIds\x3d17259,24472,24923,25260,25901,25907,26095,26446,26530\x26sugexp\x3dldymls\x26xhr\x3dt\x26q\x3dStack%20Overflow\x26cp\x3d6\x26pf\x3dp\x26sclient\x3dpsy\x26aq\x3df\x26aqi\x3dg4g-o1\x26aql\x3d\x26oq\x3dStack+\x26gs_rfai\x3d\x26pbx\x3d1\x26fp\x3df97fdf10596ae095\x26tch\x3d1\x26ech\x3d1\x26psi\x3dj9iHTO3xBo2CONvDzaEO12839712156911",
    d: "\x3cscript\x3eje.pa(_loc, \x27rso\x27, \x27\\x3c!--m--\\x3e\\x3cli class\\x3dg style\\x3d\\x22margin-left:16px\\x22\\x3e\\x3ch3 class\\x3d\\x22r hcw\\x22\\x3e\\x3ca href\\x3d\\x22http://blog.stackoverflow.com/category/podcasts/\\x22 class\\x3dl onmousedown\\x3d\\x22return rwt(this,\\x27\\x27,\\x27\\x27,\\x27\\x27,\\x275\\x27,\\x27AFQjCNGnAJSxUa4GLcg-E7PNvIFmPC53gQ\\x27,\\x27\\x27,\\x270CDEQFjAE\\x27)\\x22\\x3epodcasts - Blog – \\x3cem\\x3eStack Overflow\\x3c/em\\x3e\\x3c/a\\x3e\\x3c/h3\\x3e\\x3cdiv class\\x3d\\x22s hc\\x22\\x3eJoel and Jeff sit down with our new community coordinator, Robert Cartaino, to record a “bonus” podcast discussing the future of \\x3cem\\x3eStack Overflow\\x3c/em\\x3e and Stack \\x3cb\\x3e...\\x3c/b\\x3e\\x3cbr\\x3e\\x3cspan class\\x3df\\x3e\\x3ccite\\x3eblog.\\x3cb\\x3estackoverflow\\x3c/b\\x3e.com/category/podcasts/\\x3c/cite\\x3e - \\x3cspan class\\x3dgl\\x3e\\x3ca href\\x3d\\x22http://webcache.googleusercontent.com/search?q\\x3dcache:JT0sWmmtiAEJ:blog.stackoverflow.com/category/podcasts/+Stack+Overflow\\x26amp;cd\\x3d5\\x26amp;hl\\x3den\\x26amp;ct\\x3dclnk\\x22 onmousedown\\x3d\\x22return rwt(this,\\x27\\x27,\\x27\\x27,\\x27\\x27,\\x275\\x27,\\x27AFQjCNErCiLBch55HA8i5BAdChcmQYH8nw\\x27,\\x27\\x27,\\x270CDMQIDAE\\x27)\\x22\\x3eCached\\x3c/a\\x3e - \\x3ca href\\x3d\\x22/search?hl\\x3den\\x26amp;q\\x3drelated:blog.stackoverflow.com/category/podcasts/+Stack+Overflow\\x26amp;tbo\\x3d1\\x26amp;sa\\x3dX\\x26amp;ei\\x3dj9iHTLXlLNmXOJLQ3cMO\\x26amp;sqi\\x3d2\\x26amp;ved\\x3d0CDQQHzAE\\x22\\x3eSimilar\\x3c/a\\x3e\\x3c/span\\x3e\\x3c/span\\x3e\\x3c/div\\x3e\\x3c!--n--\\x3e\\x3c!--m--\\x3e\\x3cli class\\x3dg\\x3e\\x3ch3 class\\x3d\\x22r\\x22\\x3e\\x3ca href\\x3d\\x22http://itc.conversationsnetwork.org/series/stackoverflow.html\\x22 class\\x3dl onmousedown\\x3d\\x22return rwt(this,\\x27\\x27,\\x27\\x27,\\x27\\x27,\\x276\\x27,\\x27AFQjCNHG9l1PMbilYkhohNFuj3g6ce1LuA\\x27,\\x27\\x27,\\x270CDUQFjAF\\x27)\\x22\\x3e\\x3cem\\x3eStackOverflow\\x3c/em\\x3e\\x3c/a\\x3e\\x3c/h3\\x3e\\x3cdiv class\\x3d\\x22s\\x22\\x3eJoel and Jeff sit down with our new community coordinator, Robert Cartaino, to discuss the future of \\x3cem\\x3eStack Overflow\\x3c/em\\x3e and Stack Exchange 2.0. \\x3cb\\x3e...\\x3c/b\\x3e\\x3cbr\\x3e\\x3cspan class\\x3df\\x3e\\x3ccite\\x3eitc.conversationsnetwork.org/series/\\x3cb\\x3estackoverflow\\x3c/b\\x3e.html\\x3c/cite\\x3e - \\x3cspan class\\x3dgl\\x3e\\x3ca href\\x3d\\x22http://webcache.googleusercontent.com/search?q\\x3dcache:8MkFpx7D4wYJ:itc.conversationsnetwork.org/series/stackoverflow.html+Stack+Overflow\\x26amp;cd\\x3d6\\x26amp;hl\\x3den\\x26amp;ct\\x3dclnk\\x22 onmousedown\\x3d\\x22return rwt(this,\\x27\\x27,\\x27\\x27,\\x27\\x27,\\x276\\x27,\\x27AFQjCNFP62Bg_o2kaz3jzXxzsrTs_7RdNA\\x27,\\x27\\x27,\\x270CDcQIDAF\\x27)\\x22\\x3eCached\\x3c/a\\x3e - \\x3ca href\\x3d\\x22/search?hl\\x3den\\x26amp;q\\x3drelated:itc.conversationsnetwork.org/series/stackoverflow.html+Stack+Overflow\\x26amp;tbo\\x3d1\\x26amp;sa\\x3dX\\x26amp;ei\\x3dj9iHTLXlLNmXOJLQ3cMO\\x26amp;sqi\\x3d2\\x26amp;ved\\x3d0CDgQHzAF\\x22\\x3eSimilar\\x3c/a\\x3e\\x3c/span\\x3e\\x3c/span\\x3e\\x3c/div\\x3e\\x3c!--n--\\x3e\\x3c!--m--\\x3e\\x3cli class\\x3dg\\x3e\\x3ch3 class\\x3d\\x22r\\x22\\x3e\\x3ca href\\x3d\\x22http://support.microsoft.com/kb/145799\\x22 class\\x3dl onmousedown\\x3d\\x22return rwt(this,\\x27\\x27,\\x27\\x27,\\x27\\x27,\\x277\\x27,\\x27AFQjCNHzyj5rHEX7IiyFWnP0ziE3B32rGg\\x27,\\x27\\x27,\\x270CDkQFjAG\\x27)\\x22\\x3eHow to Troubleshoot Windows Internal \\x3cem\\x3eStack Overflow\\x3c/em\\x3e Error Messages\\x3c/a\\x3e\\x3c/h3\\x3e\\x3cdiv class\\x3d\\x22s\\x22\\x3eThis article lists steps to help you troubleshoot problems with \\x3cem\\x3estack overflow\\x3c/em\\x3e errors in  Windows. Stacks are reserved memory that programs use to process \\x3cb\\x3e...\\x3c/b\\x3e\\x3cbr\\x3e\\x3cspan class\\x3df\\x3e\\x3ccite\\x3esupport.microsoft.com/kb/145799\\x3c/cite\\x3e - \\x3cspan class\\x3dgl\\x3e\\x3ca href\\x3d\\x22http://webcache.googleusercontent.com/search?q\\x3dcache:ECO9ORCsraAJ:support.microsoft.com/kb/145799+Stack+Overflow\\x26amp;cd\\x3d7\\x26amp;hl\\x3den\\x26amp;ct\\x3dclnk\\x22 onmousedown\\x3d\\x22return rwt(this,\\x27\\x27,\\x27\\x27,\\x27\\x27,\\x277\\x27,\\x27AFQjCNHYsox9EW1Ye9Nn2G6WQzEpJDOzcw\\x27,\\x27\\x27,\\x270CDsQIDAG\\x27)\\x22\\x3eCached\\x3c/a\\x3e - \\x3ca href\\x3d\\x22/search?hl\\x3den\\x26amp;q\\x3drelated:support.microsoft.com/kb/145799+Stack+Overflow\\x26amp;tbo\\x3d1\\x26amp;sa\\x3dX\\x26amp;ei\\x3dj9iHTLXlLNmXOJLQ3cMO\\x26amp;sqi\\x3d2\\x26amp;ved\\x3d0CDwQHzAG\\x22\\x3eSimilar\\x3c/a\\x3e\\x3c/span\\x3e\\x3c/span\\x3e\\x3c/div\\x3e\\x3c!--n--\\x3e\\x3c!--m--\\x3e\\x3cli class\\x3dg\\x3e\\x3ch3 class\\x3d\\x22r\\x22\\x3e\\x3ca href\\x3d\\x22http://stackoverflow.carsonified.com/\\x22 class\\x3dl onmousedown\\x3d\\x22return rwt(this,\\x27\\x27,\\x27\\x27,\\x27\\x27,\\x278\\x27,\\x27AFQjCNHcEPoch5soLj2CpLpRfnW-Z2-aLw\\x27,\\x27\\x27,\\x270CD0QFjAH\\x27)\\x22\\x3e\\x3cem\\x3eStackOverflow\\x3c/em\\x3e DevDays » Home\\x3c/a\\x3e\\x3c/h3\\x3e\\x3cdiv class\\x3d\\x22s\\x22\\x3e\\x3cem\\x3eStackOverflow\\x3c/em\\x3e Dev Days is run by Carsonified, so please give us a shout if you need anything or are interested in sponsoring the event. \\x3cb\\x3e...\\x3c/b\\x3e\\x3cbr\\x3e\\x3cspan class\\x3df\\x3e\\x3ccite\\x3e\\x3cb\\x3estackoverflow\\x3c/b\\x3e.carsonified.com/\\x3c/cite\\x3e - \\x3cspan class\\x3dgl\\x3e\\x3ca href\\x3d\\x22http://webcache.googleusercontent.com/search?q\\x3dcache:uhl8NPgikN0J:stackoverflow.carsonified.com/+Stack+Overflow\\x26amp;cd\\x3d8\\x26amp;hl\\x3den\\x26amp;ct\\x3dclnk\\x22 onmousedown\\x3d\\x22return rwt(this,\\x27\\x27,\\x27\\x27,\\x27\\x27,\\x278\\x27,\\x27AFQjCNFf9Vl5L3FaQGPapUpIFw5gqVUCnA\\x27,\\x27\\x27,\\x270CD8QIDAH\\x27)\\x22\\x3eCached\\x3c/a\\x3e - \\x3ca href\\x3d\\x22/search?hl\\x3den\\x26amp;q\\x3drelated:stackoverflow.carsonified.com/+Stack+Overflow\\x26amp;tbo\\x3d1\\x26amp;sa\\x3dX\\x26amp;ei\\x3dj9iHTLXlLNmXOJLQ3cMO\\x26amp;sqi\\x3d2\\x26amp;ved\\x3d0CEAQHzAH\\x22\\x3eSimilar\\x3c/a\\x3e\\x3c/span\\x3e\\x3c/span\\x3e\\x3c/div\\x3e\\x3c!--n--\\x3e\\x3c!--m--\\x3e\\x3cli class\\x3dg\\x3e\\x3ch3 class\\x3d\\x22r\\x22\\x3e\\x3ca href\\x3d\\x22http://stackoverflow.org/\\x22 class\\x3dl onmousedown\\x3d\\x22return rwt(this,\\x27\\x27,\\x27\\x27,\\x27\\x27,\\x279\\x27,\\x27AFQjCNF-YrPvTLTJlFFDJrJE0cjGdlOpbg\\x27,\\x27\\x27,\\x270CEEQFjAI\\x27)\\x22\\x3e\\x3cem\\x3eStackOverflow\\x3c/em\\x3e.org\\x3c/a\\x3e\\x3c/h3\\x3e\\x3cdiv class\\x3d\\x22s\\x22\\x3e\\x3cem\\x3eStackOverflow\\x3c/em\\x3e.org began as the merging of two ideas that have been kicking around in my head for years. First, I wanted a dorky programming-related domain \\x3cb\\x3e...\\x3c/b\\x3e\\x3cbr\\x3e\\x3cspan class\\x3df\\x3e\\x3ccite\\x3e\\x3cb\\x3estackoverflow\\x3c/b\\x3e.org/\\x3c/cite\\x3e - \\x3cspan class\\x3dgl\\x3e\\x3ca href\\x3d\\x22http://webcache.googleusercontent.com/search?q\\x3dcache:u0dIlJW-XMYJ:stackoverflow.org/+Stack+Overflow\\x26amp;cd\\x3d9\\x26amp;hl\\x3den\\x26amp;ct\\x3dclnk\\x22 onmousedown\\x3d\\x22return rwt(this,\\x27\\x27,\\x27\\x27,\\x27\\x27,\\x279\\x27,\\x27AFQjCNHcJcV2QVybr6voztyPwHCrNOOD1w\\x27,\\x27\\x27,\\x270CEMQIDAI\\x27)\\x22\\x3eCached\\x3c/a\\x3e - \\x3ca href\\x3d\\x22/search?hl\\x3den\\x26amp;q\\x3drelated:stackoverflow.org/+Stack+Overflow\\x26amp;tbo\\x3d1\\x26amp;sa\\x3dX\\x26amp;ei\\x3dj9iHTLXlLNmXOJLQ3cMO\\x26amp;sqi\\x3d2\\x26amp;ved\\x3d0CEQQHzAI\\x22\\x3eSimilar\\x3c/a\\x3e\\x3c/span\\x3e\\x3c/span\\x3e\\x3c/div\\x3e\\x3c!--n--\\x3e\\x3c!--m--\\x3e\\x3cli class\\x3dg\\x3e\\x3ch3 class\\x3d\\x22r\\x22\\x3e\\x3ca href\\x3d\\x22http://embeddedgurus.com/stack-overflow/\\x22 class\\x3dl onmousedown\\x3d\\x22return rwt(this,\\x27\\x27,\\x27\\x27,\\x27\\x27,\\x2710\\x27,\\x27AFQjCNFYQ5E8irNUCpRsbOHHyfc0oqGpWw\\x27,\\x27\\x27,\\x270CEUQFjAJ\\x27)\\x22\\x3e\\x3cem\\x3eStack Overflow\\x3c/em\\x3e\\x3c/a\\x3e\\x3c/h3\\x3e\\x3cdiv class\\x3d\\x22s\\x22\\x3e\\x3cem\\x3eStack Overflow\\x3c/em\\x3e. Nigel Jones. Nigel Jones has over 20 years of experience designing electronic circuits and firmware. (full bio). Pages. Contact Nigel. Links \\x3cb\\x3e...\\x3c/b\\x3e\\x3cbr\\x3e\\x3cspan class\\x3df\\x3e\\x3ccite\\x3eembeddedgurus.com/\\x3cb\\x3estack\\x3c/b\\x3e-\\x3cb\\x3eoverflow\\x3c/b\\x3e/\\x3c/cite\\x3e - \\x3cspan class\\x3dgl\\x3e\\x3ca href\\x3d\\x22http://webcache.googleusercontent.com/search?q\\x3dcache:Rl_rUfEG_fIJ:embeddedgurus.com/stack-overflow/+Stack+Overflow\\x26amp;cd\\x3d10\\x26amp;hl\\x3den\\x26amp;ct\\x3dclnk\\x22 onmousedown\\x3d\\x22return rwt(this,\\x27\\x27,\\x27\\x27,\\x27\\x27,\\x2710\\x27,\\x27AFQjCNFqMjsc1pBI9JexjMSPY7wm5QLI8w\\x27,\\x27\\x27,\\x270CEcQIDAJ\\x27)\\x22\\x3eCached\\x3c/a\\x3e\\x3c/span\\x3e\\x3c/span\\x3e\\x3c/div\\x3e\\x3c!--n--\\x3e\x27,_ss);\x3c/script\x3e"
}/*""*/{
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    c: 1,
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}/*""*/

Further non-technical reading:

• Google Adwords - What is Google Instant?
• About Google Instant

As others have said, Google is instantly returning the results for the top result of what Google Suggest suggests. Therefore, if you type in fac, Google Suggests thinks you are typing in facebook. Interestingly, only the partial search term, fac, is sent to Google in the JSON request, so Google Suggest is used at both ends.

Searches are not actually performed on every keystroke. If you type fast enough, some can get missed.

I suspect this doesn't increase the load on Google's servers as much as you might think. Most of the additional searches are for very popular words when you start typing stuff in — if you starting searching for something beginning with f, it will search for facebook, if you start searching for something beginning with ju, it will search for justin bieber. Thanks to caching, these searches are not very expensive to do.