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So between the dives we talked about the first dive and I mentioned to him some really dirt simple advice that had been passed down to me by 50Fathom. Arch your back, get your knees up, squeeze your arse cheeks like your holding a golf ball, hands in front locked together and head up. I told him that you might be a little sore the first time
That's nearly it. Arms up, head back, feet and knees flat, butt slightly tensioned to flatten the legs, knees bent 90* (or so, depends on front/rear balance), feet dorsoflexed flat. You shouldn't have to "arch" your back, but I do slightly utilize my abs to flatten my back. I can tell when I'm using my lower back muscles instead of my glutes to keep my legs up. I sometimes do it when I assume the position in a hurry. My back gets hyperextended, and my whole torso "smiles" in my harness, with a U shape from my shoulders to my knees. Can't do it for long, or I get sore. I tension my abs slightly, and my back flattens out. This also has the effect of pre-loading my glutes and hamstrings, getting them in the right position to flex and lay flat.
"Body English" plays a huge role in attitude and stability in the water. Once you make the neuro-muscular connection between how flat "feels" and what it requires of the body to get there, BAM! trim happens on command, every time.
There are some individual factors, such as limited shoulder or neck mobility, which can impact how far the head can go back, and how easy valve drills are, but with regular utilization of some mobility/flexibility exercises, it gets way easier.