A lot of comments don't understand that this is a simulation of a fall during edge transition. If a rescuer falls on the edge, even a tensioned mainline effectively has slack in it because the rescuer will still fall the distance between their feet and their harness before the rope starts to catch. That is why it is correct, as in the video, to have some slack even in the tensioned lines in this simulation.

Well done Kirk, Gord and Cyril. Thank you for your continued work and support. Rescue.

Your test is jacked up... The point of the backup (belay in this case) is to catch the fall quickly before a lot of rope goes out. The belay and belay attendant do their job. The job of an "untensioned belay" is not to not do shit and just allow the main line snap out, introduce a fuck ton of force into the system and potentially blow the main line anchors or damage equipment. The integral part of the "untensioned backup" is the two eight mil prusiks, VT prusik, or whatever equipment passes the belay drop test. The point of the belay, and belay anchor (which should be the strongest of the two), is to take all of the force from the transitional fall and protect the main anchor and litter attendant by reliably catching very fast but not too fast, which reduces the max forces seen in the system to not blow the main line anchors, injure the attendant, and/or damage your equipment. After the edge transition is complete, fall or no, and the main line load is hanging in space with a constant unchanging weight, then you can keep all the weight on the main line while you simply change what was your belay to now match your main line's method of descent control, making it a two tension system for the rest (99.9%) of the lower.

Thanks for sharing this. It would be interesting to try this with a significant pendulum with the fall. If I recall correctly, the MPD guidance is that even when using two MPDs, one line should not be tensioned when transitioning over the edge (so that if you do pendulum a lot, the rope under tension would be cut, hopefully leaving the untentioned line to catch you near the end of the pendulum). Of course, try to find anchors & an edge that don't have a pendulum potential in the first place.

I believe what Kirk is doing is for the Two-Tensioned examples he is first placing the load onto both lines to equalize their tension; he is then lifting the load using the white line to the noted drop height. This would simulate for example an attendant losing control of the litter at the edge and taking a fall, the lines then coming into contact with a sharp edge. This edge transition being the most dangerous location on a raise or lower. For the Un-Tensioned Belay example he is similarly initially loading only a SINGLE line and allowing the 15cm of slack in the second line. He is then once again lifting the load to the noted height with the white line. The results are valid are highly illustrative. John Myers Olympic Mountain Rescue

I don't understand this video, they say the main is under tension, but then they identify an Orange rope as the main, none of which are under tension

Thanks Kirk for sharing this

Ok at 1:38 something doesn't make sense. A dedicated mainline would be under tension and not slack as this video shows. What am I missing here? I am not disputing that a twin tension system is superior. Just don't get why the load rope in the unshared system has so much slack in it. Doesn't seem realistic to me.

There is no practical scenario where anyone using twin rope system would or should have introduced slack above their working device. This (the case in your vid) would be assuming user error in application of rope access / rope rescue technique. Easy to blow holes in things when people make mistakes, but hardly something that should be flagged up as worthy of further testing.