I briefly touched on this in a previous post on Shawn Clement's Braced Tilt. Good golfers keep their posteriors in a protruded position through impact, while high-handicap golfers (like me) tend to lose that protrusion established at address (in other words, the belt buckle starts looking up too soon). When golfers make this error, they may indeed shift to the left leg and rotate around the left hip, but the right hip gets thrown out over the target line and the shoulders arms and club soon follow (over the top).
This improper move is often called standing up through the shot and golfers are often incorrectly told that you didn't keep your head down or you looked up too soon. Doing this creates little room for the right elbow to fall back down in front of the right hip, and thus it's harder to approach the ball from the inside. I believe golfers like me develop this habit in a few ways: 1) the illusion that this is what pros are doing when you witness a swing from the front-on view; 2) the fear of striking the ground behind the ball or taking deep, painful divots, when ironically staying in the shot and covering the ball in the proper manner will shallow out the swing.
The correct move (as Clement eloquently describes) is that the right hip moves backwards, towards the target, and slightly upwards on the backswing pivot, with the weight on the inside of the right leg and right instep. This creates a "tush line" that is parallel to the target line with only the right hip is touching it. Now the key point. On the downswing, the left hip ALSO moves backwards to join the "tush line," while also shifting left and turning, such that the posterior remains protruded through impact; this protrusion is not lost until late in the followthrough. The right knee will kick in more behind the left knee instead of moving out too much toward the target line.
Moving the hips this way makes it easier to stay in the right posture through the shot; the chest will cover the ball and you will have more room for the arms to swing down and out. You will also notice that you finish your swing still in the correct posture and spine angle, as if laying one's head on a pillow. Done properly, there's no way to come over the top.
Shawn Clement, the Canadian golf instructor, starts off by demonstrating how one should perform a backswing hip pivot movement - by wiping the right buttock against the glass window in the direction of the target. That's a good swing thought for a beginner golfer who has difficulty performing a "correct" backswing hip pivot movement.
Note what he states about starting the downswing's lower body movements. He recommends that one keep the right buttock back where it was at the end of the backswing (at the tush line), and he recommends that one should think of pulling the left buttock back against the glass window and wiping away from the target (pivoting the left hemi-pelvis back and to the right, so that it is against the tush line). In other words, he is recommending that one should focus one's attention on the left hemi-pelvis, and he recommends that one should think of pulling the left hip back in a left hip clearing action - without allowing the right buttocks to move away from the tush line in the direction of the ball-target line.
In other words, a good "downswing initiating" mental image for a beginner golfer is to think of getting one's weight over onto the left foot followed by an immediate sensation of actively pulling the left hemi-pelvis back (away from the ball-target line) towards the tush line.
Another major area we've worked on is my hip action. Like my shoulders, my hips now turn on a steeper angle to the ground in the backswing. It feels as if I'm sticking out the right side of my rear end as I turn to the top. This move counterbalances my left shoulder turning down: If I didn't stick my rear end out, that steep shoulder turn would put me out on my toes. On the downswing, it's all about my left hip—actually, the left knee, thigh and hip. I want to feel them clearing out, or turning to my left, so my right side can drive hard. From the top, my hips used to thrust toward the ball, which dropped the club too far to the inside and led to pushes and hooks. Now I think about pushing my left hip out to left field and then turning it behind me (above). That keeps the club coming in steep so I can really pinch the ball off the ground.