What is a Belaying Device?
A belay device acts as a brake on the climbing rope by applying friction to it. The device, plus the belayer’s quick “braking hand” (which locks off the free end of the rope), helps keep tension on the rope and helps protect the climber at the other end.
“Belaying is a variety of techniques climbers use to exert tension on a climbing rope so that a falling climber does not fall very far. A climbing partner typically applies tension at the other end of the rope whenever the climber is not moving, and removes the tension from the rope whenever the climber needs more rope to continue climbing.”