Machine...good points. It's 430 pounds, btw.
First, it's the angle. The shouders need to be behind the bar as you said.
No, I don't rock, I use a "grip and rip," tactic. So I never "set up" for the lift the way you might.
Powerlifters have as much, if not more, need for joint safety as anyone. When I say "lockout" I don't mean locking out the joints. I mean performing the lift, and showing control in the completed position, when I say "locked out." I use a "soft lock" for everything. If I don't, I feel it right away. This goes for squats too...I start slightly bent and end slightly bent. I should've been more clear about how I define lockout.
Most of my training partners pull conventional and are stronger that way. Depends on what you train for. I used to pull conventional. My conventional is about 50-60 pounds lighter than my sumo now.