What are Spring-loaded folders and applications?

Here's a video of spring loaded folders in action on OS 8 (yes, OS 8, but the principle is exactly the same):

http://www.youtube.com/watch?v=F9kdAxGe9SE

I am completely mystified as to why it has never been properly implemented in Windows. You kind of get a half-assed version of it in the folder pane of Explorer, but it's not the same as being able to 'spring' folders in the main pane.

They're things where you can drag something over them, hold it there for a second, and they'll open automatically. Some examples:

• Folders and sidebar items (and the path bar, I believe) in Finder are spring-loaded, so you can drag a file all the way around a directory structure without having to stop at each folder; it's quite useful. I'm assuming that the reference to spring-loaded windows is about the fact that Finder windows will bring themselves fully on screen and to the front if you drag something over them.
• Stacks are also spring-loaded.
• If you drag any window to the edges of the screen and wait, you'll switch between Spaces. This isn't exactly spring-loading, but it's similar.
• Xcode actually has spring-loaded tab-style segmented controls for dragging things between panes, which is quite awesome.
• Safari doesn't have spring-loaded tabs, but in my opinion it definitely should. ;)

The paradigm crops up all sorts of places in Mac OS X. It's usually handy when you're moving things.