Scenario: Your team needs to descend a tricky 4th class section that’s about 1 pitch long. The most skilled climber on the team feels fairly confident they can safely down climb it . . . but they’d feel better with some sort of backup.
A Crafty Rope Trick (CRT), sometimes called “downleading”, can be used to protect the last climber in this situation.
To begin with, one end of the rope is fixed to an anchor and the other end tossed down. Now, all but the last 2 climbers descend the fixed rope, by rappelling the single line or by downclimbing protected by a friction knot.
Here's what happens next.
The second to last climber places a few pieces of protection (snow or rock) as they descend, and then clips the rope to the pro. (This person placing the gear should of course have enough experience to make good placements, and they need to be aware to put the gear in below any tricky moves, not above.)
The last person, typically a more skilled climber, unties the fixed rope, then ties the rope into their harness.
A climber at the bottom puts the last climber at the top on belay.
The top climber then heads down the pitch cleaning gear as they come to it, just as if they were “seconding” a route. The gear prevents the last climber from a big fall should they come off.