When you power a block that is two above a piston it extends but doesn't retract after removing the above power and won't until you update the surrounding blocks.

How to copy this.

1. Take some blocks and place them 4 blocks above the ground so that you end up with a 2x2 platform 4 blocks above the ground.
2. Place some redstone on a block and then place a torch on the block beside it.
3. place a piston facing downwards with one block between the 2x2 platform.