Pre-rigging a rappel (known by some as a “stacked” rappel) is a common technique with guides and clients. But, it has some benefits that crossover well to recreational climbers too.
What is a pre-rigged rappel? It’s when everyone on the climbing team (or, as many of them as can comfortably fit near the anchor) attaches to the rope at the same time with an extended rappel. Learn about the extended rappel in detail here. The extended sling allows the people waiting to rap to be connected to the rope, but not be pulled around by the rope tension from the person on rappel.
An extended (or stacked) rappel setup. Purple sling is rapping first, while black sling is also ready to rap the moment purple goes off rappel. (Yes, eagle eye, the yellow carabiner needs to be locked . . .)
Okay, got it. So . . . why would you want to do this?
A pre-rigged rappel improve safety.
We always give our partner a safety check when we’re climbing up. Why not do the same on the way down? With everyone pre-rigged at one time, teammates can give each other a proper safety check of BRAKES (Belay device, Rope, Anchor, Knot, Extension and Safety backup).
Compare this to the traditional rappel method, where the last person waiting to rap rigs up completely on their own, with no one there to give them a safety check. This risk can be increased when an inexperienced rappeler is the last person heading down, which is typical in a two person team - the most experienced person would almost always go first, to decluster the ropes, find and rig the lower anchor, or to give a fireman’s belay to the second person down. That’s a reason why guides like this method.
Here’s one other interesting safety consideration. If the first person down ties one end of the rope into their belay loop, there is no way that they can rappel at the end of the two rope strands. This is because the second person waiting to rappel at the top has the rope essentially locked off in their rappel device. This means that the first person down does not have to tie a knot in the end of the strand, which is one less thing to worry about potentially getting stuck.
A pre-rigged rappel improves speed.
I’ve read a bunch of articles and web discussions on a pre-rigged rappel, and curiously no one seems to mention this benefit. You get everyone down the rappel faster. Why is this? Because it eliminates the downtime of waiting for climbers to rig the rappel one by one, because several people can rig up at the same time. After rigging up, everyone can be on the tensioned rope without being yanked around because of the extension. When Rapper 1 is on rappel, Rapper 2 is putting on their autoblock knot. The moment the Rapper 1 goes off rappel, they quickly feed a couple meters of rope through their device and Rapper 2 can head down immediately. The movement of climbers down the rope is pretty much constant, with no waiting for someone to rig. (Of course this downtime is much less with skilled climbers, but it can be an eternity with a larger group of beginners.)
To be fair, let's look at the potential downsides. In my mind they are small and are outweighed by the benefits, but let's talk about them anyway.
Downside #1 - The first person down can’t do a test pull to see that the ropes pull smoothly, because the person above is essentially locking the rope in place. If you have any doubts about pulling the ropes, you might not want to use this method, because pulling the rope without issue is more important than adding a small degree of speed and safety by pre-rigging.
Downside #2 - If the first person rappelling were to be somehow incapacitated by rockfall, ice fall, whatever, and be hanging their weight on the rope, the person on top would be unable to exit the system without actually cutting their leash. (Another reason why you should consider always caring a tiny knife.) Also, they probably couldn't go anywhere, because they can’t rappel on a weighted rope. Granted, the chances of this happening are extremely remote, but it's maybe worth a mention.
Should you use a pre-rigged rappel every time you rap? No, probably not. But, if you find yourself with an inexperienced partner, or with a larger group, it can add a margin of safety and save you some time.
Here is a very nice instructional video from Seattle mountaineers that covers how to set up the extended rappel. Start the video at 4:50 to see how a pre-rigged rappel is set up with two people.