Scenario: you're rappelling down a cliff when you realize your rope doesn't reach the bottom, so you'll have to ascend back up. Here's how: Detach your autoblock from below your ATC. Lock off your ATC (If you don't remember how, go back to the rappelling chapter and visit the Stopping on Rappel section). Use your autoblock to tie a friction hitch (either a Bachmann or a Klemheist) above the ATC. Tie the end of the rappelling rope to your harness. Tie a figure 8 on a bight about 3.5 feet from the end of the rope and use a carabiner to attach the loop to the friction hitch. Then tie a foot loop (Alpine Butterfly knot) in the rappelling rope about 3.5 feet below the friction hitch. Stand up in the loop to take weight off the ATC. Undo the lock off and cinch up on the ATC. Then transfer your weight to the ATC and slide the friction hitch up the rope as far as you can reach while simultaneously lifting the foot that is in the footloop. Then stand up in the foot loop and cinch up on the ATC. Repeat about a thousand times until you get to the top.
One hand has to remain on the brake line of the ATC at all times.