Minecraft wiki entry for Pulse, associated text: Each input pulse pushes an item from the dropper to the hopper next to it. When the dropper is finally emptied its comparator will turn off, allowing the item in the bottom-left hopper to move to the right, starting the reset process. When the top hopper has finished moving items back to the dropper, the item in the bottom hoppers will move back to the left, ending the reset process.