I'm curious as to how this (which I know as a "French Prussik") could possibly be jammed. IME, and that of the guides who recommended it to me, this is far less likely to cause jamming problems than a traditional single or double prussik.
For ascending, I believe their recommendation was to use a traditional single prussik for the harness, and this for the leg loop. This is certainly easier to move up an untensioned rope than a traditional prussik, but may also slip slightly more readily, I think.
Personally, I've used it mainly as a backup when abseiling; I can't understand why it's not universally used there (attached to leg-loop and below the abseil device) - you just keep your hand on it and it will allow descent, let go or allow it to move up and you are stopped.
For ascending, I believe their recommendation was to use a traditional single prussik for the harness, and this for the leg loop. This is certainly easier to move up an untensioned rope than a traditional prussik, but may also slip slightly more readily, I think.
Personally, I've used it mainly as a backup when abseiling; I can't understand why it's not universally used there (attached to leg-loop and below the abseil device) - you just keep your hand on it and it will allow descent, let go or allow it to move up and you are stopped.