An autoblock (or autobloc or "third hand") is a rope device used in climbing and caving for both rappelling (downward) and ascending (upward). While rappelling, it slides freely down the rope when pushed downward by the hand, allowing a controlled descent, but jams in the event of a sudden drop or loss of control, stopping the descent. This prevents uncontrolled falls in the event of an accident in which the abseiler loses control of the rope. For ascending, it likewise can be pushed up the rope manually when unweighted, but jams and holds when weighted by the body.