What's a 2×4 ultra clock?

Question

In this video, the uploaders describe a mechanism to "automate" the mining of trees by filling blocks around and above tree you mine with air, while accelerating the speed at which "random ticks happen". At 0:59, the video mentions that the contraption is powered by a "2×4 ultra clock."

What's a so-called "2×4 ultra clock"? The weird thing is that the actual enclosed by the command blocks quite clearly is 2×2×1...

Answer 1

They meant 2x2.

What they call "Ultra clock" is also known as a fill clock: one command block does a /fill command with redstone blocks, another one does one with a different kind of block (e.g. stone). The rest of the command blocks will now get activated 20 times per second (each game tick).

The /fill command is currently only available in the snapshots.