I'm not a GM expert, but I've always done them with my back arched and kept tight throughout the lift, with more of a pivot at the hip. So really a lot like a stiff-legged dead with the bar on my back. I just go as deep as my hamstring flexibility will allow. Correct me if I'm wrong, but I thought it was dangerous to round your back on this exercise? Unless you were specifically doing rounded-back GMs, but I wouldn't do those with PR weights that you have to struggle with.