Minecraft wiki entry for Hopper, associated text: While a locked hopper does not push or pull/collect items, it may still receive items from dispensers, droppers and other hoppers, and may have its items pulled out by another hopper beneath it. Hence, the item flow in a horizontal hopper pipe may be stopped by locking just one of the hoppers, but stopping a vertical hopper pipe requires locking two adjacent hoppers at the same time, such that both the pushing of the top one and the pulling of the bottom one are stopped.