Minecraft wiki entry for Dropper, associated text: In addition to the methods above, droppers can also be activated by quasi-connectivity.‌[Java Edition only] A dropper activates if one of the methods above would activate a mechanism component in the block above the dropper, even if there is no mechanism component there (even if the block above the dropper is air or a transparent block), but only when the dropper receives a block update (including a redstone update within two blocks of the dropper).