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I believe I read that you want to clench your butthole as well (in addition to covering nose and mouth) to prevent the force of water rushing up your cavities f

• KGIII 9y ago
  Well, sort of. I don't recall that being specifically mentioned but we were taught to tighten all muscles. You actually want to be as tense as you can when you hit the water. You want to hit it as close to straight as you can. When you do hit it, just relaxing will fold you up - by the way. You're going so fast that you won't relax before you're completely submerged and slowed. I forget the numbers but it means your dive is more shallow than if you'd stayed tensed. There's a ratio for it, but I've long since forgotten. I suspect there's been a bit posted about this. My enlistment was a long time ago but I'm told they still teach it. There may be some modifications to it, or new data for survivability.