Autoblock

An autoblock (or autobloc or "third hand") is a rope device used in climbing and caving for both rappelling (downward) and ascending (upward).

[1][2] 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.

[3] For ascending, it likewise can be pushed up the rope manually when unweighted, but jams and holds when weighted by the body.

It is made using a friction hitch around the rope, connected by a carabiner to the climber's harness, and may be combined with other climbing equipment for further safety.