Minecraft wiki entry for Creating_a_challenge_map, associated text: This trap takes advantage of the very fast speed you can achieve by sprint-jumping in a two-block high passageway. First of all, build a two-block tall passageway of whatever length you want. Put a pressure plate at the beginning of the hallway, and connect it to pistons that will retract the floor. Put in a delay, so that the floor will retract just after a sprint-jumping player gets through. If they aren't fast enough, they fall to the bottom and from there, it's up to you what happens to them...