Minecraft wiki entry for Pulse, associated text: Each input pulse pushes an item from the left dropper to the right dropper. When the left dropper is finally emptied its comparator will turn off, allowing the item in the bottom-left hopper to move to the right, starting the subtraction 1-clock driving the reset process (although the subtraction clock will pulse the dropper, the circuit's output will only alternate in signal strength, staying on the whole time – subtraction clocks can be tricky that way!). When the right dropper has finished moving items back to the left dropper, the item in the bottom hoppers will move back to the left, ending the reset process.