When you power a line along a stack of pistons, it extend(bud) the pistons underneath, and extend the right pistons on other places.
it works in a pattern and because of instant updates from extending piston heads.
Here's a pic: (I'ts hard to upload for me here)
https://www.dropbox.com/s/xspaqltw1blwlol/2013-02-13_23.09.58.png?m