Original: As the climber moves on the climb , the belayer must make sure that the climber has the right amount of rope by paying out or pulling in excess rope . If the climber falls , then they will free-fall the distance of the slack or unprotected rope before friction applied by the belayer will start to slow their descent .
Simplified: If the climber falls , the device lets the belayer also hold the rope still , so the climber hangs in the air but does not hit the ground .