There will probably be many occasions in a climber's lifetime when they will need to abseil: accessing a sea cliff climb, getting down from the top of a climb or being forced to retreat through a variety of circumstances.
Sadly a great number of tragic accidents still occur from abseiling. The most common causes are anchor failure and abseiling off the end of the rope. Some accidents are a result of genuinely bad luck but many, it seems, might be avoided. Abseiling is essentially a very straightforward technique but it is this simplicity that often invokes a slightly casual, but potentially dangerous, attitude.
Abseiling's simplicity often invokes a slightly casual, but potentially dangerous, attitude
The Figure 8 descender is the most commonly used device and one that is specifically designed for abseiling. There are many other types too, some of which incorporate a safety locking mechanism in case the abseiler loses a grip on the rope. The climber must consider carefully the worthiness of carrying an extra device that in truth might only be used for abseiling.
Belay devices do, in general, work very well as abseil devices. Some styles generate an enormous amount of friction that makes smooth descent difficult. This may be a great comfort when facing the prospect of some grotesquely terrifying abseil into a bottomless void but it can put extra strain on the anchor as you bounce jerkily downwards.
Commando-style abseiling is fine for commandos but it's not advised for climbers. The extra strain put on the anchors is considerable and though most of the time one would have reasonable anchors there will be occasions when there may be some doubt. To make the descent smoother you can place an extra karabiner between the belay device and the main screwgate attachment. Always use a screwgate 'krab' for your attachment to any abseil rope or, if you don't have one to hand, use doubled snap links, back to back.
Figure of 8 device
If you find, part way down the abseil, that you don't have enough friction and the rope is becoming increasingly difficult to grip, try taking a turn in the controlling rope under your thigh. This will be very effective. If you want to stop for any length of time you can take repeated turns around your thigh to lock off the device.
Karabiner brakes are seldom seen these days but they are a useful alternative if you lose your belay device, they do however need careful thought in setting them up. It is too easy to set them up incorrectly and in use they might actually fall apart. In an ideal world they should be constructed with screwgates, but failing the availability of sufficient screwgates can be done with snap links. As with all techniques, it's best to find somewhere safe to practice before putting them to test in a real place.
The Italian hitch might also be considered a useful alternative. Be warned though, this method is not suitable for double ropes. It's okay for fixed single ropes but as climbers rarely find themselves in that situation, it is of limited value. People often remark that it is possible to abseil with an Italian hitch by using a separate krab for each rope. Sure, it can be done but all that happens is that the two ropes plait together below you and it becomes a nightmare to unravel. It is important to use an HMS or 'pear shaped' karabiner for Italian hitches.
There are times when it's prudent to use some kind of safety back up. There are two main options. Put a French prusik knot on the rope above you, attach it to your harness via a quick draw and slide it down with you as you descend. It is important that you use a French prusik or a knot that can be released under load and that the attachment to the harness is sufficiently short to allow the knot (known as an auto-bloc) to remain within arms reach even when loaded. If the back up system comes tight, out of arms reach and you are out of contact with the rock it will be very difficult, if not impossible to release it.
The second, and perhaps the best option, is to use a French prusik attached to the harness leg loop. The prusik must not come tight up against the abseil device when it comes under full load. (If this happens it will simply not grip the rope as the device prevents it from tightening). Secondly, ensure that you have enough wraps around the rope. With soft 7mm cord, four or five turns is usually enough. Always attach the prusik to the leg loop with a screwgate (or doubled snaplink karabiners). As you descend, slide the prusik down in your controlling hand.
Another useful form of safety and one that should perhaps be used more frequently in consideration of your partner's safety, is simply to hold the bottom of the abseil ropes loosely. If your partner has a problem on the abseil, pull really tightly on the rope and they should come to a halt.
If you are joining two ropes together to abseil on, make sure that both ropes are pretty equal in length. If the abseil finishes any distance above the ground, tie a big, chunky knot in the two ends. Remember which rope you have to pull to retrieve both the ropes after the descent. One way to remember is to mark the pulling rope with a quickdraw attached to it - the other krab can be fixed to your harness so that it is pulled down with you as you descend. When joining two ropes together use a double fisherman's knot, double Figure 8 knot or, for ease of pulling over a flat edge, an overhand knot. If you use the latter be sure to tie the knot at least 50cm from the ends.
Inevitably there will be occasions when you come across fixed abseils threaded with gnarly looking 'tat'. If you have the slightest doubt about its safety, replace the tat with something of your own. It is not worth risking all for the sake of leaving a bit of gear behind. Nylon deteriorates rapidly in UV and even slings that have been in place for only a few months can be substantially weakened. Check too that the slings have not been melted by numerous ropes being pulled through them.
Do not assume too readily that if someone has used the abseil before you that it will be just as safe when you come to use it. Different climbers have differing perceptions of what is safe and what is not.
Always check before you go over the edge ... harness buckle, abseil device, safety back-up, anchor, knot in the end of rope, no tangles, rope reaches the ground or ledge, rope can be retrieved okay ... now you can go!