We can see the hips turning during golf swing from slow motion video: very fast from beginning of downswing to impact,  having a pulse at impact zone and then continuing to turn naturally to stop. 

Many instructors explain this process by these follow different ways: 

1. The hips accelerate for a while, then decelerate as energy moves into the upper body, then accelerate again as they are pulled around to the finish by the upper body, arms and club.  
2. What you see in this swing is the hips clearing to make room for the arms, then waiting for the club, then being pulled through by the momentum of the swing. The power comes from the speed of the club that pulls the hips to their finish position.
3. It is  the classic hip stall, it’s a reaction to something else going wrong…. you’re going to block it or slice it so you bail on the shot…. or quite simply your balance was off in the backswing.

They can guess so, but can you make the hips really accelerate, stop and accelerate again in one second of downswing?  The scientific reason is clear: you should turn hips and spine as fast as possible during downswing, impact and follow through without any stalling. But the key is the whole body will lose balance if there is no related reaction to offset the hips turning. The needed reaction is the opposite turning of the lower body — the right leg’s opposite turning will make the hips stall at impact — this is the basic natural action: conservation of angular momentum ( See detail at Chapter 6).      

Therefore the hips stall during the impact zone is a good sign for a good swing. It is the still point between the upper body turning one direction and the lower body turning opposite direction.    

Remember, the right leg’s reaction of turning oppositely is a natural action, you just let it happen.  What you need to do is simple:

1. Help a little of the right leg’s opposite turning and do not make the right heel off the ground during the impact zone;
2. Don’t care too much about the position of belt buckle during impact. Some instructors let the belt buckle facing the ball at impact, but you know it is not suitable for everybody just like the top position,  because everybody has his own body structure and his own way to keep balance at impact. If you watch the slow motion of the pro swings, you can find the belt buckle positions are different: some facing the ball and some passing the ball;
3. The hips turning stall is not for every shot, especially for the short distance. For swing with a short club, the hips do not need to turn very fast for the club speed, because what the golfer needs is accuracy instead of distance.