Minecraft wiki entry for Hopper, associated text: Item pushes and pulls are processed in the same game tick, but pushes are processed before pulls. In the schematic, the empty hopper first pulls an item from chest A as it cannot push anything into chest B. After the cooldown, the hopper first pushes its item into chest B before pulling another item from chest A, both pushing and pulling in the same tick, and the process repeats. The hopper stops pulling when A is empty, and stops pushing when B becomes full.