Minecraft wiki entry for End_of_light_mob_farms, associated text: The end of light mob farm involves 2 key components - update suppressors and light suppressors using an update suppressor to prevent the portals from updating when the side obsidian blocks are removed. This creates a sliced portal in which mobs can spawn where the light level is 0. The light suppressors allow the light levels of the portal blocks to be 0, meaning that mobs can spawn directly into the portals and get transported into a drop chute.