Minecraft wiki entry for Block_update_detector, associated text: By eliminating the reset mechanism of a BUD switch, it becomes a T-BUD or Toggle-BUD. This device has two stable states, which it switches between when it detects a block update next to the piston. This is equivalent to a normal BUD connected to a T Flip-Flop, but much simpler to build. It is useful for tracking the state of blocks like furnaces, grass block, dirt, and beds. However, it has useful capabilities for placing some blocks two blocks away. When placing a piece of redstone dust two blocks away, the T-BUD activates when it is destroyed only. When placing a repeater two blocks away, the T-BUD activates only on the placement.