Minecraft wiki entry for Secret_door, associated text: It is possible for a redstone output to be generated as a result of a block being destroyed. To achieve this, place a redstone torch on the side of a block facing away from you. Next, set up an inverter between the block and your mechanism. When the block is destroyed, the torch will fall, the lack of input will be inverted, and the mechanism will activate. Alternatively, an Observer may be used.