This lazy block then functions as the "sensor." When an adjacent block changes state in some way, the game updates all adjacent block including the sensor block and "wakes it up." The circuit then performs some other action and, with most designs, goes back into the sleeping state.This BUD switch is originally in-game and it was added in 1.11 Snapshot 16w39a with name 'Observer'.By forming a feedback loop, where the state of the piston controls the state of its power source, it becomes the sensor of a simple BUD switch: In this design, the piston will only extend in response to an adjacent block update, but after extending it will immediately retract because the arm of the piston notices the repeater next to it turning off.This is how a piston BUD resets itself: by having the power run next to the extended piston arm, with a delay.Note: If the block diagonally adjacent to the sticky piston becomes horizontally adjacent, the circuit will constantly pulse instead.This design can be extended with multiple sensor blocks. Alternative method: if you take a sticky piston facing parallel to the ground, then place a Block of Redstone on its face, and connect the Block of Redstone out to the side of the piston through a repeater that goes into a block diagonally adjacent to the piston, it works a fully functional BUD in 1.5.
This BUD switch takes advantage of the fact that water flows toward the nearest hole, but it doesn't notice new holes until the water is updated. This design is outdated and is more complicated than newer designs.Many BUD switch designs exploit a property of pistons called quasi-connectivity.