Minecraft wiki entry for Block_update_detector, associated text: By using a waterlogged block that is not allowed to flow (through the use of trapdoors or other techniques) and an observer clock, if the player updates the waterlogged block rapidly and the observer clock is in the same chunk the clock will break and give a constant output out of both sides. Then whenever the block is next updated, one side of the clock will pulse off for one tick. You can create a BUD switch using this by linking up using both sides as the output of the BUD.