Minecraft wiki entry for Instant_repeaters, associated text: The most common instant repeater requires a pulse at least four redstone ticks long. It is made by having a redstone signal powering a block that powers a sticky piston on top of it, facing the direction of the rest of the signal. This sticky piston has a redstone block on its face. The block that is powered is also attached to a sticky piston, but it's facing the opposite direction and is diagonally below the redstone block, such that the block can power the top piston when the bottom piston is extended. The rest of the signal is powered when the redstone block is extended forward.