In reply to needvert:
> (In reply to David Coley)
>
> Would one do that via threading rope through metal anchor point, tying end to harness, and putting the grigri on the other strand?
>
> [Meaning you're suspended by a growing loop of rope]
There are a couple of 'lowering yourself' type options, but its generally seen as a pretty bad idea to have a moving loaded rope passing through the anchors, (a really bad idea if you're abbing direct off a tape cluster or something). It will also knacker the rope pretty quick too.
The preferred method I've done and seen is to tie a double fig 8 loop at the mid point, thread the anchor till you reach the knot, then clip a screwgate from the loop to the opposite strand, which is also the strand you ab on. So you're effectively larks footing the anchor. You retrieve by pulling the side with the knot on. If you have a tag line or even a load of tapes, you can move the knot closer to one end, daisy chain the stuff on the pulling end thereby increasing your ab-able distance.
If its not a steep ab or there's ledges, the knot and crab can catch on all sorts. You can do it without the crab allegedly, but then you've got rope on rope on the pull and the friction that goes with it.