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
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.
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 biners 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 biner slows your descent. This may be counterintuitive, as it seems that the sharper angle made by a single biner 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) If you have an ATC, Reverso or similar auto-locking belay device, clip the “ear” to your belay loop.
Doing this tilts the rappel device back towards you, making a sharper angle for the exiting rope, which slows your descent.
Note: You definitely need to practice this in a controlled setting. If the top carabiner is too long, it won't bend the ATC and you will get zero additional friction. If the top carabiner is too short, it's going to bend the ATC too much and pretty much lock up your entire rappel so you can't move. The size and length of carabiners used with this setup is critical, so practice ahead of time.
Note: This setup does not work if you have an extended rappel. If your rappel is on anything other than your belay loop and you try this, and you will instantly lock up your rappel device, turning it into an ascender. Which, depending on the circumstance, might be exactly what you want to do. Again, practice this on flat ground to see how it works.
3) 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 biners clipped to rope”).
4 - Try the “Rappel Z”.
This is a slick method that adds even more friction to method #3. Do the same as method #3 above, but then add a locker (or opposite and opposed carabiners as shown below) onto the rope above your belay device and redirect your brake strand through that locker.
It's a much better show than a tell, see the image below.