Minecraft wiki entry for Block_update_detector, associated text: An extremely compact one-time design that is easily hidden and therefore ideal for traps is as follows: Place a piston, a sticky piston is most useful, parallel to the ground. Place a block one block to any side except the piston head and one block above the piston. On the side of the block facing the piston, place a redstone torch. Then place a lever on any other side of the block and switch it on. The redstone torch will go out but the piston will remain extended. You can then break everything except for the piston, but make sure to break the lever after you break the torch. A block update will then cause the piston to retract, possibly pulling the floor out from under the victim. Remember, though, that after the detector is tripped, it has to be rebuilt to be used again.