Minecraft wiki entry for Block_update_detector, associated text: Or, for it being smaller: The player can make a 2×1 (3×1 when extended) BUD by having a 2 sticky pistons (stacked) both facing any direction except up and down, then on both of those sticky pistons add Redstone Blocks. Whenever a block is updated, (destroyed, placed etc.) it will switch the bottom piston outwards. Good for traps, as the BUD may need to be compact. The only con to this is that it has to be reset.