Redstone on a Piston

It clearly show it in this video:
http://www.youtube.com/watch?v=HUHS4Ps1gWE&feature=youtu.be

If you can't or dont have time to watch it. It is basically a piston clock that rapidly pushes a block back and forth and when a piston is placed inbetween it is pushed triggering a piston behind it to be pushed under a redstone without removing the redstone.