Passing a knot on a single rope rappel

The following tip is from the excellent book, “Climbing Self Rescue” by Andy Tyson and Molly Loomis.

If you know you will be descending two single strand ropes tied together prior to loading the line, here is a useful variation.  (This technique will not work as described with double ropes; it can only be implemented on a fixed single-strand line.)

1) When tying the two ropes together, leave an extra foot of tail on the side exiting closest to the ground. Tie a figure 8 on a bight (or overhand on a bight) in that tail.

2) When you arrive at the knot you need to pass, simply clip into the knot tied on the tail with some sort of tether. This becomes your primary attachment to the rope.

3) Move your rappel device around the knot, reattach it below the knot, unclip your tether, and continue with your rappel.

Notes:
- The last person to rap should untie the figure 8 loop, to prevent possible snagging when the rope is pulled.

- This technique is easier on lower angle terrain where you have some sort of a foothold you can use while you are passing the knot. 

Why, you might ask, would I ever rappel two fixed single-strand ropes with a knot in the middle? 
If your climbing team has a long section of steep but down-climbable terrain (snow or rock) to descend, and you have two ropes, this can be the fastest way to get everyone down. Tie the ropes together in the manner described above, and fix one end of the rope to an anchor. Everyone but the last climber raps on the single strand and passes the knot.  The last climber (maybe the most skilled) unties the rope, tosses it, and downclimbs.

Or, if it’s a rescue situation and/or you simply need to get off the terrain as quickly as possible, even the last person can rappel and you simply leave your rope behind. (Obviously not preferred, but perhaps occasionally necessary.)