Minecraft wiki entry for Enderman_farming, associated text: It is possible for the user to gain EXP from this trap, as the player hitting a mob once registers the mobs' death as a player-kill. This means that even with the pistons crushing the mobs down to half a heart, you can still give experience from the player hitting the mobs at their point of death. It is also possible to switch the levers and pressure pads with a button, these can be timed to crush the endermen, then release when they reach the health points of half a heart, readying the endermen for a one hit kill from the player.