To easily switch from a rappel into rope climbing, you will need an autoblocking device from which you will rappel on an extension. This will require you to girth-hitch a sling through the tie-in point on your harness. Clip a locking carabiner to this extension and then run your rope through your autoblocking device the way you would normally rappel. The actual device will then be at chest or face height.
Extended rappels are extremely useful for a variety of reasons. They make it less likely that any clothing will get caught in the device, they ensure that your autoblock back-up is completely incapable of touching the the device, and they allow for tricks like the one described here.
To convert your rappel device into a rope climbing device, simply clip a locking carbiner to the "fin" of the autblocking device. From there, you will have to stand up on a small ledge in order to clip the fin to your belay loop. Once this is clipped, the device will autolock. If you pull rope through the device to climb up, it will automatically lock off.
If you are on lower-angled terrain and are able to climb up the face, then you will not need to do anything more than to pull the rope through the device as a self-belay. However, if you are on steeper terrain, you may be required to add a foot prussik. This should be added to the rope above the device.
Photo by Zeph Locke
Once you have climbed back up the rope for whatever reason you needed to climb back up the rope, then you can easily revert the system back into a rappel by unclipping the device from the fin. Once you've unclipped this, your system will once again look like an extended rappel and you will be able to descend.
While rope tricks are rope tricks, sometimes they can be valuable. I have often used this particular trick to release a stuck rappel rope or do a variety of other things on the cliff. The more rope tricks that you know, the more tools that you have in your toolbox...and a big toolbox of tricks and techniques is arguably the best way to be ready for anything...!
--Jason D. Martin