What is "This Country" ? Why do you assume we know the same people ? You make so many assumptions.. My primary training decoy is from a club in France and he brings many of his friends to Canada to train. We train around Toronto and Sudbury primarily.
Lifework was a typo for bitework, was replying on my phone, my apologies. You will have to take it or leave it that there are others using x-backs for bitework. I don't name drop as a habit and peoples names do not belong on public forums without their permission by my standards. But god forbid we use a harness designed for endurance work by dogs who can actually breathe properly during the work!! A proper fitting x-back puts the side straps along the side of the dog, back towards the hips, so good low leash handling gives you better control of the entire dog than agitation harnesses (again I should clarify we are ring people so we do leg work primarily).
I'm glad you know a lot about physics but I don't think you know anything about training sled dogs.
Harnesses for sledding get way more workout than an agitation harness ever would. Training programs for sledding involve training different paces to build endurance and strength. Speed is set by the driver and a team of yearlings will continuously throw themselves at their harnesses until they learn to just lean into them and pull full bore. You have an entire team doing this straining against each other. The gang line and the harnesses form a chain and a chain is only as strong as the weakest link.. This work and straining happens daily for hours in the case of distance teams. Harnesses last years, sometimes decades in conditions where they are wet and then frozen. Also, again, not everyone training bitework lets their dog slam at the end of the harness. We do mostly tension work and drag ins with the line, and then use a bungee for slowing down targeting and for reflex work on the guard, etc.
It's crazy some random person is more closed minded about using an xback in French ring training than multiple older experienced French decoys who have been to selectifs and finals for many years!