For the purpose of this post, I'm referring to the primary spine angle, which refers to the angle of the upper body to the legs when viewed down the line. The secondary spine angle refers to the axis tilt to the right when viewed from the front, primarily caused by the right hand being lower than the left hand on the grip.
The mysterious "spine angle" really isn't solely about the spine, as the S&T creators have eloquently argued, because the spine bends all over the place during a golf swing. It's not really about keeping the head down either, because you can still change the head position vertically from the neck while maintaining spine angle. I like "covering the ball" best as a description, because it drives home the point for me that the overall angle of the upper body established at address (i.e., posture created by bending over from the hips) should be maintained at least through impact; I would argue that amateur golfers should work to maintain it well after impact, as they risk coming up too early otherwise. This is that "head on a pillow" look in the followthrough and finish position, or looking at your initial ball flight through slanted eyes.
In other words, the "tush line" should be maintained through impact; if the pelvis begins to move forward (i.e., the upper body comes up)--even a little--at any point before or during impact, the radius required for the swing gets longer, making it harder to hold lag through impact when it's needed. As a matter of fact, losing the spine angle is a major factor in many mishits and bad results, to include inconsistent, short, fat, thin, pulled, and sliced shots. Most expert golfers may come up a little well after impact, but you'll notice that they are still bent over towards the target line, even in their finish positions. Some professional golfers have upper body positions that actually sink even lower than their address positions before impact!
An additional, related aspect of spine angle is shoulder plane. The left shoulder should point down towards the ground (in the direction of the ball) at the top of the backswing (right shoulder rotated up and behind the head), maintaining a perpendicular relationship to the spine angle. The left shoulder should be touching or very close to touching your chin at the top. On the downswing, the right shoulder should replace the left shoulder's previous position, and vice versa. Keeping the shoulder plane more "vertical" allows the arms to hang more from the shoulders as they should through the shot. The correct shoulder plane is actually a side bend; stand up and bend fully sideways to the left from the waist. Now simply rotate to the right (pointing in between both feet) and the shoulder plane should be in the correct position to maintain the backswing spine angle.
Believe it or not, the right knee can affect your spine angle. Notwithstanding Stack and Tilt, you should strive to keep a bend in your right knee at the top of your backswing.
Maintaining one's spine angle or covering the ball is important for EVERY shot in the book, to include putting, chipping, pitching, and bunker shots. It encourages movements that make drives long, powerful, and accurate. It encourages you to keep lag through the shot, because otherwise you will hit fat. It helps prevent fat shots due to an an early release or cast (usually a direct, autonomic, and unconscious manipulation as a result of "standing up"). "Standing up" and "casting" are directly related to each other. Staying over the ball frees you up to swing faster while still feeling in control; you can really pop the driver when covering the ball.
Actually, the only way you can really deliver lag to the ball is to maintain the spine angle. Now you'll have to learn how to lag the club by keeping the wrist angle late into the ball to keep from hitting fat. You will have to learn to keep the wrist cocked late and make contact with the ball with the right wrist still feeling bent back or dorsiflexed. If you lose the spine angle or stand up then you must cast or straighten the wrists to have any hope of contacting the ball; the result in this case is a powerless poot shot, duff shot, or skull across the ground!
If the takeaway isn't your problem (or even if it is), do the same procedure above with the only exception being to stop at the top of your backswing. Again, any distance between the ball and club represents how much you came out of your spine angle. Now, slowly rotate to your usual impact position and do the same procedure. In each case, you will learn exactly how much your spine angle is changing throughout the swing. Do the same procedure again for takeaway, backswing, and downswing and pay attention to your pelvis; it's very easy to see when you're standing up when you pay attention to it. Video analysis also works, or you can have a friend touch a club to your head while you work through these exercises.
You may only be standing up in one part of the swing, or if you're like me you could be doing it some throughout the entire swing. It's possible to do well at first but start losing the spine angle late into a round as one begins to get tired. The tendency is always to stand up, because it requires strength and endurance in the core muscles to keep covering the ball as one must to execute good shots.
Now, it's important to drill the correct feeling, which is literally a feeling of covering the ball with your chest through impact. If you've been standing up, you'll notice a distinctly new feeling of the right should coming down and through the shot and replace the position of the left shoulder as the upper body rotates through.
The whole idea of keeping your spine angle consistent is that it will help you to keep the “shape” of your swing consistent. When the shape of your swing is consistent, it is much easier to deliver the club face in a square position at impact.
Newsletter: The 3rd Key To Consistency: Spine Angle | Target Centered Golf
There are two main ways to lose the spine angle. A dive and a loss of “tush line.” A dive is what Tiger does and to see if you have this problem, put your finger on the end of your nose on a video from behind and down the line. If your nose moves out or down toward the ball, that is not good. If it happens a little, it’s nothing to worry about, so don’t go out and ruin your swing by trying to keep you head still.
To see if you are losing the tush line, you need a vertical line (same camera angle as above) like an imaginary wall your behind is up against and your body pulls away from that line during the swing. This is also known as standing up or pulling out of it…not to be confused with keeping your head down.
Maintaining spine angle | Monte Scheinblum's Blog
One crucial element for good iron play is to maintain a constant spine angle throughout the swing. This enables you to hit down at impact, correctly taking a divot after the ball. Many amateurs tend to rise up as they swing the club down in a mistaken attempt to help lift the ball into the air. Lifting leads to poor contact such as fat or thin shots. To correct this tendency, swing to a finish position and then hold it for a moment. Then bring the club back down as if someone hit the rewind button. You should be able to get right back into your address position. That's maintaining your spine angle.
How to play consistent golf: Golf Digest