Minecraft wiki entry for Headless_pistons, associated text: This technique can even be used to break bedrock or other unmovable blocks. This includes barrier blocks, which can make it difficult to make retaining walls for players on vanilla servers. First, place a sticky piston adjacent to the block you want to break, with the head end facing it. Next, place any movable block on the other side of said block. Finally, unpower the piston. The block on the other side replaces the target block and the headless piston gets its head back, as normal.