Minecraft wiki entry for Piston, associated text: This design pushes and pull a block two spaces instead of one. The first and second repeaters must be set to delays of 2 and 4 respectively. The pistons are sticky and the device correctly pushes and retracts the block. The main trick is properly sequencing the retraction because the back piston does not pull back the forward piston when it is extended. Additionally, the back piston retract only the forward piston, not the block. To handle these issues the forward piston must be retracted, pulled back, then extended and retracted again.