Let the coaches give a real answer. On the deadlift, I believe Rip has said many times that you'll definitely see truly strong men with some rounding in the back and a more dynamic start, but that the weight doesn't come off the floor until the hips have come up. I believe Robert Santana has written that he thinks that this happens because the lifters are trying to start the weight up with a perception of momentum. Doesn't mean that it's a good idea for people who aren't top-level lifters . . .