How to safely shorten your tether
At a busy anchor, especially with distractions, it's quite easy to clip your rappel tether / PAS incorrectly.
I’ve done this myself, and fortunately realized my mistake very soon after, yikes!
The main cause of this problem is often when changing the length of your tether.
A common way to do this is to completely unclip the carabiner from one loop, and then clip it into another.
When you do this, you introduce the chance of clipping it wrong.
Here are two recent accidents where this seemed to be a direct cause. (Both of these reports are from the American Alpine Club’s website called The Prescription, which offers a monthly blog and archive of North American climbing accidents.)
Here's the analysis of one accident from the American Alpine Club:
Laycock’s accident was eerily similar to another recent incident, suffered by a climber in Arizona. Both fallen climbers had tied overhand knots in a 120cm length loop of 20mm sewn webbing to create adjustment pockets for a home-made PAS. This is a common practice. In both cases, it appears that the tether was not clipped correctly with the carabiner, but instead the knot caught in the bottom, non-gated end of the tether carabiner.
Wow, that photo is scary! You can see that clipped like this the knot MAY jam into the carabiner temporarily, even under light bodyweight. This could easily fool you that you are properly connected, when in fact you're absolutely not!
Here's a simple way to pretty much eliminate this problem: keep a locking carabiner as your primary connection to the anchor, always on the end of your tether. To shorten your tether, don't move that carabiner at all. Instead add a second one and use that to adjust the length of your connection.
Clip a locking carabiner to the end of your PAS. (I like a double action twist lock carabiner such as Petzl Sm’D. Once you let go of the gate, you know it's locked.)
Clip the locker to the anchor.
Clip a small carabiner to the sewn loop closest to your harness. Here I'm using a non-locker, but if you prefer a locker, go for it. (The more scared I might be, the more I might want to use a second locker. =^
To shorten the tether, simply clip the non-locker to any other loop, or to the locker that’s on the anchor. Boom, you’re now closer to the anchor without ever opening the primary locker.
A few general notes on tethers . . .
This method does not apply if you are using a Petzl Connect Adjust or similar adjustable tether.
This method applies if you're using a sewn sling with maybe a few overhand knots tied in it, or a sewn loop style PAS (Personal Anchor System).
Here I'm using the term “PAS” in a general way to cover sewn loop tethers made by many different manufacturers. I'm pretty sure Metolius was the first, and they might have a trademark on the name.
It's good practice to NOT keep any sort of tether permanently attached to your harness. You probably only need it when you’re rappelling, so keep it on the rear gear loop until it's time to head down.
Many climbers prefer to use a 120 cm sewn sling as a rappel tether. This works just fine and is useful for lots of other things. If you tie knots in your sling to give multiple clip in points, this same method works.
Old school aid climbers have been using this technique for a long time with sewn pocket daisy chains, which you’re hopefully NOT using for a tether. These are designed for aid climbing, bodyweight only, not as a rappel tether.
Are sewn loops PAS’s . . . passe? I'm not gonna get into that argument here; some people love ‘em, some people don’t. No matter what tether system you use, this method of adjusting your position with a second carabiner can pretty much eliminate the problems described in the above accident reports.