Minecraft wiki entry for Block_update_detector, associated text: Many BUD switch designs exploit a property of pistons called quasi-connectivity. A piston can receive power through a space one block above itself. In this case, the power source is either above and to the side of the piston, or two blocks above it. However, when powered in this way, the piston does not immediately notice changes in the state of the power source. When some other block update happens next to the piston, it will "wake up" and react to the change. 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: