Minecraft wiki entry for Randomizers, associated text: When the input turns on, the dropper will randomly choose to push either the stackable item or the non-stackable item into the hopper, causing the comparator to output either power level 1 or 3. Because the powered dropper is a solid/opaque block, it will also deactivate the hopper, preventing it from pushing the item back to the dropper until the input turns off.