I've recently been playing around with starting with my hips really high and legs straight and lowering my ass into the sweet spot before pulling up. My shins are uber vertical like this and the reps are fast and awesome. If you look at my recent video I did the opposite of this, got real tight with knees forwards and lower hips that rose into the sweet spot. Didn't work as well tbh
I can't think of a single heavy pull that's done without a rounded back unless the guy has an Olympic lifting background. A lot of people will say to keep the back flat in training and not worry if it rounds come meet time and I think that is silly tbh. Why would you practice one technique and compete in another? So I now lift with the back rounded all the time, locked into position (it doesn't move or flex) as this is the position it will lock into when I do a max pull anyway.
Very good video