Minecraft wiki entry for Quasi-connectivity, associated text: There are two primary methods of designing a torch key. The first is to place a block update detector under the ground so that the placement of a redstone torch updates the BUD. However, BUDs can also be updated remotely by other redstone components, increasing the chances of discovery. The second method is to use immediate QC activation by placing the torch so that it simply activates a piston by quasi-connectivity.