Minecraft wiki entry for Trap_design, associated text: False walls (pistons/gravity). False walls hide important parts. This is a complication in that it does not deal damage, but hides the method, thus giving the target a false sense of security. Pistons can easily place and replace false walls, but redstone wiring can be difficult to hide in an open area. Sand and gravel can be used as an airlock to reveal important parts by using water to knock down the supporting torch, but this requires manual reseting every time the trap is triggered.