A full shoulder turn (90 degrees)--with hips that turn only to 45 degrees--has been described as a top backswing fundamental. I believe that, when performed the correct way (see Pull vs. Push below), it may very well be THE top fundamental for the full swing and pitch shots.
I feel that making a complete shoulder turn is one of the most important fundamentals in the golf swing. I think between having a good grip, a smooth tempo and a full shoulder turn, you can really play solid golf--and that's without thinking about "swing positions" or anything else!
So what do I mean by a full shoulder turn? I mean that on a full shot, strive to turn your shoulders 90 degrees. Your lead shoulder (left shoulder for a right-handed golfer) should be under your chin, and your back should be facing the target.
Check to see if your back is turned to the target. When you make a full 90-degree shoulder turn, your target should be directly behind your back.
Most golfers know that a full, 90-degree shoulder turn is a crucial element of a solid golf swing. Without it, a proper weight shift and a correct swing plane are almost impossible to achieve. A good shoulder turn not only ensures that your shoulders and chest are behind the ball at the top of the backswing, but helps maintain consistent balance throughout your motion. Before you can master a proper shoulder turn, however, it’s important to understand what it entails and exactly what it is.
[From Golf Tips - Turnstyle]
How does one know what "correct" looks like? Examine any professional golfer at the top of the backswing in the front-on view. Note in the photo the lines drawn on Ernie Els' beautiful backswing. The line drawn on his right shin is angled back towards him and towards the target; also, note how the line separates from his waist, demonstarting that his right knee is well bent at the top, ensuring no hip over-rotation and no sway to the right. Then notice the line drawn on his left lat; it is angled in toward the right leg and away from the target! This is a good solid pivot behind the ball, with no sway and no reverse pivot. If you can draw these lines on your backswing at the top with the same relative angles, you have a solid pivot.
Push vs. Pull
The correct takeaway or early backswing feeling is to "pull" the right shoulder 90-degrees straight back until it's behind one's head, with the left shoulder just going along for the ride, until it's behind the ball; the incorrect feeling is to reach over with the left side, or "push," to get the left shoulder behind the ball. It's as if there's a wall just outside the right side of the golfer's torso (not legs), and there's nowhere for the right shoulder to go except straight back. This is the correct shoulder rotation; don't even think about the left shoulder...it will follow. A really cool result of this line of thinking is that the upper swing center (the middle of your chest) will NOT move off the ball, and when you shift your weight into the left leg to start down, the upper swing center will be just in front of the ball, ensuring you hit ball then ground.
Chuck Quinton's Rotary Swing offers this great idea for easily achieving a 90-degree shoulder turn (even older players); I tried this, and I was amazed at how different (and great) it felt. Basically, Chuck is saying that a "pull" shoulder turn keeps you more centered and simplifies the movement, while a "push" shoulder turn moves you off of the ball. Pulling the right shoulder straight back uses a different group of muscles than the pushing the left shoulder back. And according to Chuck, this efficient movement will even take care of the takeaway. And a little digging around on the interwebs validated this concept; there are many others who are teaching this pull the right shoulder back versus push the left shoulder back method.
Performed correctly, you will get a nice, centered, one-piece takeaway, where the right arm stays above the left for most of the backswing, when viewing face-on. The wrists and right elbow seem to fold up naturally, the hands don't get too close to the right shoulder, and the left arm doesn't collapse. There should be no thought given to moving the arms, wrists, or hands to get the club started back and to the top--just consciously move the right shoulder straight back and let everything else follow. This is where maintaining the triangle formed by the arms and club head for as long as possible comes into play: the right wrist shouldn't dorsiflex backwards here (which would yank the club inside), the right elbow should not bend yet (it stays straight and above the left arm for as long as possible), and the wrists shouldn't cock upwards yet (which would shorten the backswing width). The right shoulder is pulling the stable, intact triangle assembly back in one swoop--a one-piece takeaway.
Now for the "gotchas." Doing this is a distinctly different feeling than pushing the left shoulder over and back as you may have been taught. And if you're not careful, it can lead to some other problems (this is true for any positive change you might make to your golf swing).
First, it's important to keep the right elbow pointed down at the top; don't let it chicken wing, fly, and point out behind you, as this can cause the left arm to collapse and other errors. Pulling the right shoulder back immediately doesn't mean the right elbow should do the same thing. Just focus on turning the right shoulder back and keeping both arms straight for as long as possible. Then simply allow the right elbow to fold such that the right hand goes straight up, the right elbow point pit points straight up, and the right elbow stays pointing downward. Next, be aware of the potential for the right leg to straighten too much, causing a lean to the left at the top. This can show up as a feeling of too much weight on your front leg at the top and a pull or slice ball flight. To fight it, remember that more weight should be on the inside of the right foot at the top, and the left shoulder should point behind the ball; you should start your downswing by bumping the left hip slightly and then turning left using the left lat as "center" of the downswing. This should get the path going inside-out and a ball flight starting out to the right before slightly drawing back onto target.
To get the proper shoulder turn, has a very simple solution: Turn your right shoulder back (not your left) and behind you. So many golfers have been told to turn their left shoulder back that their left shoulder might be pointing behind the ball, but the right has stayed were it was. Bio-mechanically your shoulders can squeeze together which is what most amateurs do. I call this a false turn.
The right shoulder contains arguably the most powerful set of muscles that could help start the swing and concentrating on them can override mistakes that the smaller fast twitch muscles might make. Concentrating on turning the right shoulder away from the ball helps stop excess hand action, which is one of the most common backswing faults.
When you start the club back, the right shoulder does NOT move sideways (to the right)…at all! This is a lateral movement that will create all sorts of compensations coming down.
The right shoulder moves BACK immediately!
Golf is a rotational movement, so the rotation of your shoulders early on is critical to getting to the top of your backswing in the correct position, in order to have a full golf swing loaded with power. So from now on, I want you to think of your right shoulder going back when you take the club away, NOT to the side.
Does it really matter if we pull or push? Yes, it does. People say that we should use the “big” muscles in a golf swing. These big muscles work by pulling, so pulling is bio-mechanically correct. They never tell us where these big muscles are, but they are primarily in your back.
In [Kuchar's] takeaway he gets a nice width off the ball. This gives him the proper angle coming down. You can already see how he’s turning his right shoulder behind his body.
The role of the left arm in the backswing is...pretty much nothing. Your left arm stays right in front of your chest. You'll pull your right shoulder blade back, and the left arm will elevate a little bit. That's it.
The takeaway actually starts with no wrist action at all. It should be a one piece move that is best performed by letting the right shoulder blade begin moving back.
The right shoulder initiates the swing, while the right wrist follows in unison. When this occurs, the club's handle and the shoulder move together as a unit. This is known as swinging the handle, and it widens the swing arc going back.
The start of your first move is to draw your right shoulder and armpit area back towards your right heel--in a straight line. The completion of this move will place you in a position which features: The front of your shoulders closely in line with your right foot. Your shoulder blades facing the green or fairway area where your ball will land.
Another good way to [turn your left shoulder behind the ball] is by turning your right shoulder back on the backswing. Try this next time you are on the practice tee. Make some practice swings and try turning your right shoulder directly behind your head at the top of the backswing. When I do this, it is easier to get coiled behind the ball and it is easier to complete my backswing-it feels longer.
When you turn the right shoulder back, check to make sure the left shoulder is past the ball(or to the right of the ball). If the left shoulder does not get back “behind the ball”, then you may not be shifting your weight to the back leg at the top and will not have the power coming back to the ball.
Your first move away from the ball if you’re a slider is to ROTATE your right shoulder immediately behind you! This will get you turning and not sliding to the right. Once you feel the rotation in your core, you will be able to unwind with a much higher clubhead speed.
Start the swing with the right shoulder. This keeps the hands in front of the chest and guarantees a full shoulder turn. By initiating the swing with the right shoulder, the entire body stays connected. Conversely, starting the swing with the left shoulder doesn’t necessarily turn the right shoulder, creating a myriad of potential problems, chief among them, limiting the range of motion.
Another good feel for me on the backswing is to pinch my right shoulder blade in toward the middle of my back. This is my way of keeping my shoulders turning on a steep angle and making a full windup.