Minecraft wiki entry for Enderman_farming, associated text: The spawning pads either have pressure plates on them, connected to sticky pistons with blocks attached, or a tripwire attached to normal pistons to push the endermen off the pads, down the drop and landing on the platform with only half a heart of health left, allowing for one hit kills by punching. A good way to start punching is to create a two-high ceiling and start staring at the endermen.