Petzl's Grigri is an auto-locking belay device and self-braking descender in one. To belay a climber, you slide the rope smoothly through the device. If the climber falls, the sudden pull makes the cam pivot, which presses on the rope and stops it running. For lowering the climber off a climb using a Grigri, you operate the handle so as to unjam the rope and you control the running of the rope by varying your grip on the free end of the rope. Although it provides increased safety, the Grigri is not a substitute for vigilance on the part of the belayer, who must remain attentive to the climber at all times. Great for climbing walls and climbing towers.