The QRB is the connection between the rescue man and the hoist system of the helicopter.
The QRB is able to open under tension up to 150 kg. Security wise this is a very important ability. If the rescue man get into problems, entangled on a wreckage or stock between rocks, the helicopter has a problem, and the crew could be forced to cut the cable. If this happens for a helicopter equipped with a single hoist, the helicopter is not able to get back to rescue the rescue man, and far the less the victim.