Minecraft wiki entry for Block_update_detector, associated text: Another extremely compact design involves using 1 sticky piston facing up with a slime block and redstone block stacked on top of it. This design is 1×3 (1×4 when extended) and allows the piston to be updated from 5 of its sides. It does not need to be reset after triggering, but players do need to make sure that the slime block isn't grabbing a wall in order to activate in the first place.