Minecraft wiki entry for Hopper_clocks, associated text: The top part is a regular ethonian hopper clock. Once per cycle, the block of redstone will move left and activate both of the droppers in the second stage (the left dropper is powered directly, while the right dropper is activated because it's next to a powered block: the left dropper). The block of redstone in the second stage ensures that only one dropper will actually push an item, forcing the items to move in one direction until the block of redstone moves.