Minecraft wiki entry for Clock, associated text: Design B shows how to counter block dropping with an optional, non-sticky, piston. The non sticky piston (the bottom one) is needed for the 1 tick clock as a self repair mechanism. It prevents detaching of the moving block from the sticky piston. If using it only for a 1-tick cycle, the repeater (under the extended piston) can be replaced with redstone wire. The toggle line stops the clock on a high signal.