Minecraft wiki entry for Clock, associated text: When the comparator first receives full power, it outputs strength 15 to the block in front of it, which passes the same signal strength to the dust next to it. The signal strength then declines by 1 (to 14) as it moves to the dust next to the comparator. In the next tick, the comparator subtracts 14 from its 15 input to output only signal strength 1. This is enough to barely power the block and the dust next to the block, but isn't strong enough to reach back to the dust next to the comparator, so on the next tick the comparator subtracts 0 from its input and the cycle starts again.