Minecraft wiki entry for Miscellaneous, associated text: The sticky piston should be activated by quasi-connectivity, but it doesn't know that yet. When a block next to the piston is updated, the piston "notices" it should activate, and extends. However, that pushes a block of glass into the sand's place, so the piston is no longer activated by quasi-connectivity. Accordingly, the piston retracts, resetting the circuit. This circuit outputs an off-pulse; invert the output for a regular on-pulse.