Note - This post discusses techniques and methods used in vertical rope work. If you do them wrong, you could die. Always practice vertical rope techniques under the supervision of an experienced climber, and ideally in a progression: from flat ground, to staircase, to vertical close to the ground before you ever try them in a real climbing situation.
Sure, on a bluebird day, standard rappel practice is probably going to work fine. However, some rappel situations, such as:
a single strand
wearing a heavy pack
an icy, slippery slab
a knuckle scraping overhang
not sure where the next anchors are
or any multiple combination of these cluster factors, can make a rappel backup a fine idea.
Raps under Less Than Ideal conditions are often safer and easier when you add extra friction to the rap to better control your descent speed. This can be especially true for beginners, who most of the time and are happy to go down a little more slowly under greater control.
Note: It is critically important that you always use a rappel device that is properly sized for your rope.
This is especially true if you are using twin or half ropes. Many accidents have happened when people have started to slide uncontrollably when they had a skinny rope in a rappel device Designed for something larger. The techniques below do not replace having the correct rappel device to begin with.
Here are a few easy ways to add friction to a rappel.
Note - Methods 1 and 2 are what you make call “committing”, Meaning, once they are set up, you can't really do anything to adjust or remove them until you are off rappel.
Methods 3 and 4 are what we might think of as “adjustable”; you can have the carabiners in place, ready to go in case you need them. If you find your rap is going faster than you like, you can then use them. Because rappels get faster near the bottom when there is less rope weight, this can be a good approach.
1) Use two same sized carabiners on your harness and clip your rap device and rope through both.
The extra friction of the second carabiner slows your descent. This may be counterintuitive, as it seems that the sharper angle made by a single carabiner would slow the rope more. It’s actually the opposite - try it yourself and see.
Bonus tip - if you find yourself having to belay on a skinny rope with a belay device that's not quite rated for it, you can use this same “double belay carabiner” trick to add a bit more friction. (Ideally, you should never find yourself in this situation . . .)
2) Clip a spare carabiner to your leg loop.
After you’ve put the rope through your rappel device and locking carabiner on the front of your harness in the normal manner, clip the brake strand of the rope through a carabiner on your leg loop, the same side as your brake hand. By simply moving your brake hand up (more friction) or down (less friction), you can maintain better control. See image below; (imagine it without the “opposed carabiners clipped to rope”).
3) “Rappel Z” with an extended rappel
This is a slick method that adds even more friction to method #2. Do the same as method #2 above, but then add a locker to your belay loop, another locker to your rappel device carabiner and redirect your brake strand through both lockers. This is the way to rig it if you use an extended rappel.
This has the advantage of working with an autoblock back up, if you choose to use it. Note that you need to attach the autoblock to the leg loop, and it might also add an unnecessary extra amount of friction and cluster. As always, practice in a controlled situation, like a staircase, before you use it for real.
It's a much better show than a tell, see the image below from the excellent book, “Self Rescue”, by David Fasulo.
4) “Rappel Z” from the belay loop
The same idea as number 3 above, only without the rappel extension. The image shows two opposite and opposed carabiners on the rope, you can replace this with a single locking carabiner. Be sure your leg loops are sturdy if you are doing this. It does not work with an auto block back up.