How to put a redstone block in clock mode?

I saw this video by SethBling. (The video is caught up to the moment of my question)

He says "Make sure these are in clock mode" what does he mean by clock mode?

As a note, Sethbling asks you to import the schematic. Anyways...

What Sethbling refers to as clock mode is a redstone block that is continually being replaced.

As you see in the video, as he destroys the block, it immediately appears again. This is a neat little trick - you can make a redstone clock that signals up to 20 times per second.

To do this, have a layout like this:

z-- <-    [C][R][C]    -> z++

Where Cs are the command blocks and R is the redstone block. Open up F3 and make sure that the z coordinate of the right command block > redstone block > left command block.

If you want to line it up over a different axis, just change the following codes. Remember - first x, then y, finally z.

On the leftmost command block, type this:

/fill ~ ~ ~1 ~ ~ ~1 redstone_block

On the rightmost:

/fill ~ ~ ~-1 ~ ~ ~-1 air

Finally, put the redstone block in the middle. Now, it will magically become the fastest clock in Minecraft. (As far as we know.)