Minecraft wiki entry for Trap_design, associated text: A piston smasher trap uses a piston to periodically suffocate trapped mobs. Mobs can be packed together by dropping them into the smasher. The design to the right cannot process a massive amount of mobs at once, since mobs will push against each other and clog the machine. The smasher to the right will eventually work through a buildup of mobs though. Also, this design doesn't work for spiders.