Minecraft wiki entry for Instant_repeaters, associated text: It works by BUD-powering the bottom piston. When the pulse is received, the top piston starts moving. This updates the bottom piston, causing it to retract at exactly the same time as the top one is extending, thus depowering the top piston. This 0-ticks the top piston. The redstone block is now powering the bottom piston, so it extends. The block on its face comes back to its original position, but the start of the redstone line is still powered. This powers the top piston, and retracts the redstone block, depowering the redstone line at the end and BUD-powering the bottom piston. In real time, it looks like the contraption does an "inchworm." This produces pulses that are two redstone ticks long, which means that they can be chained together.