This exploits how pistons work when placing them while moving your camera. If you flicked your camera while placing a piston it would first face one direction for a tick and then correct itself and face the other way. Now that doesn't happen but it still behaves the same. If you do that on a block where the piston will be powered for you it will have two heads. For the server and other players only one of them is there. If you break the base only the ghost piston head will remain as seen in theDe video.