Minecraft wiki entry for Block_update_detector, associated text: The Block Update Detector, or BUD switch, is a redstone mechanism that uses quirks in the game in order to detect changes in nearby blocks. There are many different designs that are all based on the same general idea: a redstone circuit is put in a state where it should be following a mechanic, but the game has not updated the block to follow this rule. This unupdated block then functions as the "sensor". When an adjacent block changes state in some way, the game updates all adjacent blocks - including the sensor block - and powers them up. The circuit then performs some other action and, with most designs, goes back into standby.