Why would a trained diver have their bungied backup way down at their chest? Even then, pulling their long hose down their neck wouldn't trap the backup reg. Think of the routing, which is behind the neck.
The diver could have the necklace tight on the neck, I don't think it matters, may even make it harder on the ''donor', as it leaves less slack in the necklace once trapped.
I think the moves would be an under the left arm, over the right shoulder hug from the left back:
- left hand moves under their left arm pit, catching the long hose going up the chest and displacing it from the left to the right side of the backup necklaced regulator.
- right hand goes over the right shoulder to grasp that long hose and keep it on the right side of the backup regulator, just for a moment. Just barely to the right is what you want.
- left hand then grasps reg from mouth and draws down and to the left under the armpit and back, coming down on the upper chest or lower neck just to the left of the backup regulator.
- back up reg should now be pinned below and down from where the two hoses cross on the chest/lower neck, pulling the slack out of the bungee.
- long hose coming up the chest pins the reg on the right via the short hose and fitting going into the reg.
- long hose going down the chest pins the first hose down and also the left side bungee.
- reg is pined tight on both sides and down on the chest.
- if the necklace is breakaway or pull out you may be able to do that and free the left side. Thus angling it up a bit to reach the mouth, or pulling the short hose to feed more length out from the upward long hose pinning it, making it easier to reach the mouth.
- long hose looped under light canister prevents getting more slack in hose.
Try it on land. I'm not sure how the BSAC diver was doing it, I'd be curious to hear. But that seems a plausible plan. Add thick wetsuits and decreased neck mobility to reach the trapped reg. Displacing the long hose to the right accidentally could... happen.
Or you could trap the backup reg body directly under just the long hose coming down, but that is iffy, and easy to pop out.
Like I said, a
targeted take. And most are taught donate, so to ask, unless in extremis.