NOTO: The only upward force generated by the hips that counts is the force that moves the bar. Your naked ass moving up all by itself does not generate any momentum against a 400 bar laying on the floor, quite happy to stay right where it is.
The reason they get in the knees-forward and consequently hamstring-loose position is because (I actually do know -- I lied earlier) it is easier to get your low back tight like this than when you have your hamstrings tight. The problem is that it rolls the bar way forward of the mid-foot. A hips-up position tightens the hamstrings first, and has the benefit of allowing the mid-foot bar placement, but forces the lifter to practice tightening the low-back under hamstring tension. A hips-up position also reduces variability in the bar path off the ground because the bar path from this position is essentially vertical, and this usually produces less bar path variability going into the second pull above the knees. But the real advantage is that if the bar comes off the floor right over the mid-foot, energy is not wasted on the lateral movement of the bar that gets it there, there is no lever arm between the bar and the mid-foot until the lateral motion gets it back over the mid-foot and returns the lever arm to zero, the more open knee angle puts the quads in a more efficient position to work on the knee, and there is a shorter lever arm between the bar and the hip when the bar path is vertical. This is why I teach this start position to Olympic weightlifters, but is the source of a lot of argument since few other coaches do it this way. And an essential part of pulling the bar is -- recognized by even hard-headed coaches -- keeping the bar as close to your torso on the way up as possible. Why not get it as close as possible off the floor, so it has a greater likelihood of being there and thus staying there, since it has to be over the mid-foot to be in balance anyway? We do it like this here, and it works perfectly.