Definitely not a duplicated question to this one... Rather a much more specific question...
The situation arises when there are bolted anchors at foot level on the edge of the cliff:
The issue is that if you have set up the anchor off a cordelette (or a sling) as in...
and you clip a dynamic rope to this anchor, and then set up the rappelling device (extended ATC with Prusik knot)
you may very well find yourself hanging from some 4 to 5 feet of slack.
If you can't set up a static rope in the back of the ledge (say, off a tree or boulder) to lower yourself smoothly until the rappel device comes under tension, I only see two solutions to avoid downclimbing on a slack rappel setup with a possible nasty slip:
Set up a static rope off one of the bolts (using one of the locking biners already in use for the main anchor), and use a grigri to lower yourself until the Prusik engages the ATC and makes the rappel device taut. This would entail connecting a dynamic system and the static rope to a single biner, which is probably not a good idea.
Perhaps using a personal anchor system. And this is the question. Good idea? And this is the question... Good idea? Can the Petzl device below (with progressive lengthening options) avoid a bad fall with a huge fall factor? How can the PAS be disengaged after placing the rappel system under tension?
... using the Petzl personal anchor system: