Friction is very important in rock climbing. If there was not friction, one would not be able to rappel or belay.  Friction from the belay device assists in bleeding off some of the energy from a fall.  When a person falls, they have potential energy, which is transferred into kinetic energy as the person falls. This energy needs to be absorbed somewhere in order for the climber to survive the fall.  Humans can survive no higher than 15 times their weight. Most of this energy is absorbed through the rope.
    Ideally, a belay device can dissipate up to 4kN of force from a fall.  This varies with the rope age, size, angle of pull, and loading (Hattingh).  If the force is greater than the frictional force of the belay device, there is rope slippage.   When a belay device is used properly, a belayer can dissipate more energy, while falling, from rope slippage. By letting some rope slip through the belay device, it can absorb some of the energy from the falling climber.  Rope slippage lengthens the rope, which allows the rope to stretch, absorbing more energy from the fall. This energy created by the falling climber is mostly transferred into heat. You can tell this because the belay device become really hot afterwards.  This table below shows the braking effect of four major types of belay devices.




    This table shows the Gri Gri as having the greatest braking effect with  >4kN.  The GriGri is great at stopping falls, but it does not allow for any rope slippage. What is great about the GriGri is it locks up automatically, much like a car seat belt, when a sudden force pulls on the rope.  The bad thing about the Grigri is that if a climber has a serious fall, meaning from a great height, the climber comes to a sudden halt absorbing more of the energy.  Increasing the deceleration time reduces the force. The Gri Gri has no deceleration time, which creates a high force that acts on the climber, rope, and anchor system.

    There are several devices that can be used to belay a climber. They all work using the same principle, by creating a frictional force to stop a fall or to bring someone to the ground.  The rope passes through the belay device, then the free arm with the loose, tail end of the rope is swung back, preferably behind the back to provide the most possible friction on the belay device. The greater the angle between the two parts of the rope, the more frictional force there is in stopping a fall. The diagram below will, hopefully, explain more and give you an example.

  
 From the left side down these belay devices are: Pyramid, Reverso, B-52, and GriGri. From the right side going down they are: Sticht plate, ATC, Jaws, and Figure 8 (Luebben 61).