Minecraft wiki entry for Defense, associated text: The said slime will travel through the corridor, and every time it lands on a pressure plate, it will be struck by two arrows, and if or when it splits, the process will simply repeat itself. To collect the drops, simply remove the pressure plates, stroll on through (reloading as you go if needed) and then when you are done, simply replace the pressure plates on the way out. to help prevent mobs entering while you do this, it may be useful to place a door at the entrance or exit in the wall, so you may close it when you are in the corridor and open it to begin the executions. This also works for most other mobs that will fit through the corridor.