Minecraft wiki entry for Miscellaneous, associated text: The block update detector switch, or simply BUD, detects any time an adjacent block receives an update. An update is anything that changes that block's state: block placed, destroyed, door opened, repeater delay changed, cake eaten, grass growing, snow falling, furnace used (or turns off), and so on (chests opened and crafting tables used do not cause updates, sleeping in a bed does). When a block is updated, all adjacent blocks are also eligible for an update, and this can be used to produce a redstone signal or for other purposes.