Imagine a nail without head that has been fully hit into a piece of wood. Reversing the action doesn't pull the nail out unless you weld the hammer to the end o

Imagine a nail without head that has been fully hit into a piece of wood. Reversing the action doesn't pull the nail out unless you weld the hammer to the end of nail. It can't be pulled out either because there is nothing sticking out from surface and can't be pushed from other side because it's blocked.