People who arch their backs upward is chating themselves. I see many white folks do it alot. I know they can't handle their max, but are willing to anyways even if their form sucks. I don't focus too much on form. Only on those exercises that will injured you like bending backs, dead lift, military, and others. For me, I felt most strongest at doing the monkey bars and it really give me more strength than the regular machine pull ups.
Post a vid of your workouts. Would like to see them.