Minecraft wiki entry for Pulse, associated text: This design uses a trick to limit the output pulse to a single tick. A comparator can't produce a 1-tick pulse by subtraction from an exterior source (such as if the repeater was set to a 1-tick delay), but if the external source would usually produce a 2-tick pulse or more, the comparator can short-circuit itself into a 1-tick pulse by incorporating it into a subtraction 1-clock (the block and parallel dust after the comparator), but only allowing the clock to run for one cycle.