This tip is from my expert canyoneering friend Kevin Clark, thanks Kevin!
A “courtesy anchor” is a concept from the canyoneering world, where generally a LOT more thought is given to rappel technique than is typical in rock climbing.
A courtesy anchor is not something you’ll rig very often, but in certain situations it's a most clever trick to have in the toolbox.
Imagine this scenario - A rappel anchor has a long extension on it, maybe webbing or chain. The purpose of the extension is to hang the anchor master point out over the lip of a rock ledge, to make an easier to pull the rope. The problem is, this probably will make an awkward start to the rappel, because the master point is hanging out into space rather than being on on the actual anchor, which is set back on a nice flat ledge. (In the Portland Oregon area, the rappel off of Rooster Rock in the Columbia river gorge is a great place where you can use this technique.)
The “courtesy anchor” concept is a simple solution to this. The main benefit: only one person on the team (ideally the person with the most experience, who goes last) has to perform the awkward rappel start. Don't make everyone on the team perform an awkward / dangerous / difficult hard start.
There are various ways to rig this. Here’s one of them.
Add a carabiner onto the original anchor point, and then bring the master point (here a quicklink) back and clip it to this added carabiner.
A simple way to add this second sling is with a basket hitch, because it's quick to rig, plenty strong, and easy for the last person down to quickly remove.
This allows every person but the last to start their rappel on a nice flat ledge rather than shimmying over the edge. The last person to go, hopefully a more experienced and skilled climber, removes the courtesy sling/biner, extends the master point to its original position, and makes the awkward start to the rappel.
The last person has a few options to increase their safety.
Have a firefighter’s belay from below
Lock off their rappel device with a mule knot
Tie a “catastrophe knot”, an overhand loop with both strands of the rope, about 10 feet below the edge, and clipping that to their belay loop
Put a klemheist knot or some other friction hitch that is releasable under load above their belay device
Or some combination of the above.
If all goes as planned, this makes a faster and safer rappel for just about everybody. Except the poor suckah’ who has to go last . . .
Here’s how it works.
Original anchor: Webbing tied around a tree with a rewoven overhand knot. Overhand on a bight at the master point. Quicklink at the master point. Rappel rope through quicklink. This anchor is great for pulling the rope because it extends the master point over the cliff edge, but it’s not so easy to start your rappel.
Basket hitch another sling (yellow) onto the tree, and clip it with a locking carabiner. (If this sling is higher up on the tree, it makes it easier to start the rappel. But this also increases leverage, so only put the sling higher up if the tree is stout.)
Finally, clip the quicklink to the locking carabiner. Doing this allows everyone except the last person to start their rappel right at the anchor. The last person removes the sling and carabiner, extends the master point to the edge of the ledge, and performs the somewhat awkward rappel start, leaving the rope in a better position for a successful retrieval.