Minecraft wiki entry for Block_update_detector, associated text: The following designs works because pistons can't be pushed by other pistons while extended. Also, when a piston retracts, it doesn't notify any other pistons that were trying to push it. The piston with the block attached to it is sticky and acts as the sensor. This design has the advantage of a low profile, and also that the quirk it exploits is less "buggy" and less likely to be fixed in the future.