Most climbers know intrinsically that there are a few of different methods that a climber might use to belay from the top. These are indirect belays, redirected belays and direct belays. Following is a quick rundown of each:
An indirect belay is when one belays directly off of his body. In the old days a climber would finish a line, clip into the anchor and then put his follower on belay directly off his belay loop. This is referred to as an indirect belay because the belay doesn't directly transfer force into the anchor. The force must go through the belayer's body first.
Very few experienced climbers still use an indirect belay for standard rock climbing. However, in a setting where one cannot build a strong enough anchor, it makes a lot of sense to put your body between the force of the load and whatever anchor you have.
In the above photo, AAI Guide Tad McCrea is belaying directly off his harness on a steep slope. His is attached to a snow picket, but a snow picket isn't that strong. In a snow setting, an indirect belay allows one to absorb some of the force so that it's not directly transmuted to the anchor.
Certainly, if it is impossible to build a solid rock anchor, a stance with a single piece could be almost as good as a bombproof anchor.
The biggest downside to an indirect belay is escaping the system. It's reasonable to tie-off a system and transfer the load to the anchor using some rock rescue trickery. However, if you put your body between the anchor and the load to begin with, your anchor may not be good enough to take the load...which could be a problem.
In the 1990s, it became quite popular to climb a pitch, clip into the anchor and then redirect your belay off the anchor point and back down to the climber, essentially making a mini-toprope. Often one would redirect off a single piece in order to make sure there was enough room to belay.
Climbers found this to be much more pleasant than your standard indirect belay. They liked the idea that they would be pulled up instead of down when a person fell.
There are a few problems with the system. First, when a climber belays with a redirect, there is a pulley-effect, which doubles the force on the anchor. This isn't a very good idea if you're using this on a single piece or have a weak anchor. Second, if the climber is heavier than the belayer, the belayer can get pulled up into the redirect and potentially let go. And third, this is a hard system to escape in the event of an emergency.
Modern climbing technology has nearly eliminated the redirect belay from use. There are very few circumstances where this technique is applicable.
The direct belay is a belay directly off the anchor. These are the most common belays in the climbing world today. Most climbers use an autoblocking device, like a ATC Guide or a Petzel Reverso, but one could also belay directly off the anchor with a munter-hitch.
The idea behind a direct belay is that, (1) you are not in the system; and (2) it's very easy to escape the belay. If you can build a solid anchor, there is almost no reason to use anything but a direct belay.
--Jason D. Martin