Minecraft wiki entry for Hopper, associated text: This design relies on the fact that, inside of Minecraft's code, hoppers search for an item to be taken from the input side before outputting an item to another container. This results in the bottom hoppers taking the filtered item(s) before the items currently running through the storage system are passed over to the next hopper. However, because hoppers have a 0.4 second cooldown after transferring an item, bear in mind that both the input and output will activate should there be more than 1 item available to pass. For example, if 2 iron ingots were in the same hopper, 1 of them would get taken by the hopper beneath, while the second one would get outputted to the side and continue along the sorting chain.