Minecraft wiki entry for Randomizers, associated text: The hopper is held deactivated by the powered dust when the circuit is off, and by the powered dropper when the circuit is on. But, when the input turns off, there is a brief 1-tick moment when the dropper has just turned off, but the torch attached to it hasn't turned on again. This allows the hopper to activate for 1 tick, pushing an item back into the dropper. A hopper always pushes items from its left slots first, so the hopper will push back the item the dropper pushed into it, rather than any of the 16-stackable items in its far right slot, allowing the circuit to reset itself.