Learning to belay is the single most important thing about beginning to climb. Belaying is what keeps climbing safe. The techniques used are incredibly simple and very important. The first rule of belaying is never take your hand off the brake strand. If the climber falls and you are not holding the belay strand one of two things will happen—you will either drop them and they will deck or you will catch the rope while giving yourself a massive burn. Both of these outcomes are bad—but one can literally lead to death. This cannot be stressed enough—never take your hand off the brake strand.
Before you belay, you need to establish a form of communication with your partner—you need to be sure you will be using the same verbiage. Miscommunications can lead to the climber having difficulty advancing up the rock, getting stranded mid climb, or even serious injury. You also need to make sure you know how to properly feed the rope into your belay device—all devices will involve a bight of rope creating friction on the device.
There are different techniques depending on whether you are top rope belaying or lead belaying. But they both involve the same action to arrest the fall. In order to lead belay, you will feed rope through the device while the climber ascends the wall. When they reach a clip, you will need to feed a few extra feet so they can safely clip in. However, you do not want to give them too much; because if they fall before clipping they will fall double the distance to the previous clip plus the excess slack. Lead belaying is a balancing act between keeping falls short and giving the climber enough slack to freely climb.
Top rope belaying uses the exact opposite technique. Instead of feeding rope, you will be continuously taking in slack as the climber advances up the wall. You will want to leave a small amount of slack so the rope doesn’t help pull the climber up the wall, however, if the climber gets stuck you can give them a slight boost by taking in all the slack and relieving some of their weight.
But regardless of which style of belay you are using, you arrest a fall in the same way. In order for the belay device to generate enough friction to stop the fall, you need to hold the brake strand below the device. At any time you are not feeding or taking in rope, you should keep the brake strand below the device. This will help to ensure the rope does not slip through your hand and give you a nasty burn.
Belaying is the most important aspect of safety in climbing. Reading a how-to is no substitution for being taught by an experienced climber. And remember, never take your hand off the brake strand.