Minecraft wiki entry for Clock, associated text: Design F is an unusual, stable, 1-tick piston clock. Unlike most repeater-based 1-clocks, its signal is fast enough to make a sticky piston reliably toggle its block, dropping and picking it up on alternate pulses. For the clock to work, the block the piston moves must be placed last. The piston extends and retracts quickly. The output wire appears to stay off, because it's changing state faster than the game visually updates. However, attaching a redstone lamp, dispenser, dropper, piston, etc. to the output shows that it is working. The clock can be turned off by a redstone signal (e.g. the lever shown on the block below it) to the piston.