The speed at which top lifters get under the bar is amazing. The final pull in either lift is pulling the body under the bar.
When the feet are off the ground, and the arms pulling, the body must go under.
I don't know what you mean by "diving" under and I can't see why the speed with which one goes under would effect the trajectory of the bar.
"Morning, Putski eats it, noon, Putski eats it, night, Putski eats it. Putski loves!"