Powered mechanisms react to change in redstone power level

Ok, so I was playing with redstone, suddently something weird happened. A piston (tested on both pistons, same thing) was powered with redstone and there was a pressureplate next to it. When I stepped on the pressureplate and the plate deactivated the piston quickly extends and retracks.

So exaclty how to do it:
Its so easy to do and hard do tell with words so I will just include a picture.

And this works with a normal piston and a sticky piston.

And one more weird thing is the redstone line to the piston must be five blocks or longer.
(Im sorry if I have bad english, I'm from Norway you know )