Minecraft wiki entry for Redstone_circuits, associated text: A block update detector (BUD, or BUD switch) is a circuit that reacts to a block changing its state (for example, stone being mined, water changing to ice, a pumpkin growing next to a pumpkin stem, etc.). BUDs react by producing a pulse, while T-BUDs (toggleable BUDs) react by toggling their output state. These are generally based on subtle quirks or glitches in device behavior; current circuits most often depend on pistons. As of Java Edition 1.11, many of the functions of BUDs were condensed into the observer, however, a BUD circuit can also detect other changes undetectable by observers, like a furnace finishing smelting or something being crafted in a crafting table. The addition of this was made to move toward feature parity with Bedrock Edition versions.