From my experience, what Kenny said is absolutely true because I had groin strain, which I never experienced before, from doing low bar squats and sumo deadlifts.
AFAICT using a wide stance, the adductors are used much more. Rip teaches to push the knees as far out as possible for this and other reasons: it minimizes the chance of rounding the back as the femur can clear the hip more easily, in this article (PDF)
he calls it "hip impingement" and "active hip", but the same matter is discussed in Starting Strength. Also using more muscles in synergy should help move more weight.
BTW it's kind of amazed me that a PL squat suit can add 50-70kg (110-154lb) to your squat!
I was reading just yesterday Wikipedia about bench shirts
, found this part nothing short of impressive:
(for example, Scot Mendelson, whose shirted bench press record is 1030 lbs, while his unshirted best is 715 lbs), records across different federations or categories may not be directly comparable
after watching more film of me, I see I was shifting weight to the front of my feet. Bad bad bad.
I think some amount of shift is acceptable..? when the load is challenging I can feel a good part of the weight on the balls of my feet, regardless of the habit of keeping my toes curled up all the time.
edit: forgotten link to Rip's article