What is Shoulder Turn in the Golf Swing?
To “turn the shoulders” is an instruction commonly heard by golfers.
What does it mean?
Let’s look at the image provided by my trusted source, RST Golf.
The angle of the shoulders in the backswing is called your PRE-SHOULDER TURN angle.
The angle of the shoulders in the down swing is called your POST-SHOULDER TURN angle.
People often make the mistake of trying to simply tip their shoulder forward in the down swing as if they were trying to twist themselves around like a corkscrew.
But this is not shifting your shoulders as the term is commonly used in golf.
Now, this may seem a bit counter intuitive, but in order to shift your shoulders properly, you often need to set yourself up with what might appear to be a less than ideal posture at address.
For instance, suppose your shoulders and hips are already too far square and you are fighting to help them swing to the top of the backswing.
If you now try to force your shoulders to shift outside the target line, you have to over-swing and the only way you are going to do that is by hitting up on the ball on the backswing.
Shoulder Turn and Tilt in the Backswing
These two shots show the difference between the "small shoulders" position, where the shoulders are tilted and the arms are flexed and the "big shoulders" position where the shoulders are turned away from the target.
The shoulders should turn the same way that the hips are turning, which is away from the target with a slightly increasing angle.
Ideally the shoulders should be parallel with the target on the downswing.
The "knuckles test" is a good drill for the shoulders.
Grip the shaft of the club like you normally would, but place your knuckles on the ground next to your feet.
"Keep the knuckles on the ground all the way around," advises Thomson.
As you bring the club back, notice how high above the turf the club is.
You should have the same dynamic, even turn throughout the swing, from backswing to downswing.
Your line of action will change for each shot, but your body should be flexed each time.
Your shoulder turn will help you control your ball flight.
If you don't have a good line of action with your shoulders, you won't hit your shot straight.
Shoulder Turn in the Downswing
Shoulder Turn in the downswing is opposite from the Torque Turn.
Torque Turn is the turning of the shoulders before the elbow starts to turn and shoulders stop turning before the hips and knees.
Shoulder Turn is the turning of the shoulders after the elbow starts.
I think of it this way. Torque Turn is more about bending the upper body. Shoulder Turn is more about extending the upper body. Torque Turn is quick and short with a stop at the top. Shoulder Turn is slower with a more continuous turning.
So in the downswing, Torque Turn is deliver the club down the target line and shoulder turn is deliver the top hand open to the target line.
I talk about some great drills to enhance this movement in the Simple Golf Lesson Downswing DVD.
It is really important to make sure you have this shoulder turn in your swing. In all levels of golf it is a key swing change to higher handicapper like you see in this lesson.
Shoulder Turn in the Follow Through
This is a significant component of the final phase of each golf swing and a vital component of any good golf shot, particularly shots from the rough.
The shoulder turn increases the club head speed towards the target at the instant of impact.
This action is made possible by bringing the right arm around the body towards the target during the follow through.
The right elbow should be pointing straight towards the target during the initial part of the follow through with the right forearm pointing slightly to the left to simulate the body’s rotation against the chest wall.
This begins the shoulder turn action prior to the right wrist and club face “coming around” to face the final target at impact. This brings the club head into a square position at impact.
You can practice this motion effectively with hitting wiffle balls into a net or with making full swings with at a practice ball. Make sure your shoulder turn brings your body towards the target at full speed with your right arm pointing towards the target throughout the full swing. This will maximize your club head speed at impact.
Why Shoulder Turn is necessary in Golf?
Shoulder turn is very important in golf because it provides the golfers with additional torque which yields more accuracy and distance from the tee. 
Shoulder turn is required for a properly executed takeaway, which is the first swing movement in the golf swing. Similarly, shoulder turn is also necessary for properly executed forward swing.
Some golfers do not understand shoulder turn and how important it is to the golf swing. They try to rotate only at their waist and fail to understand how the shoulders are connected to the backswing and forward swing. 
Therefore, they have to deal with a lack of power, accuracy and distance.
Turning the shoulders is beneficial in so many ways because it helps in producing efficient forward swing. In fact, the golfers who have a proper shoulder turn are able to produce a more powerful swing. 
Their arms and hands move faster and the centrifugal force generated by the swing improves the overall craftsmanship of the golfer. This is because the centrifugal force helps the golf clubs to move in the arc.
The golf player is also able to maintain body balance and posture while rotating his shoulders because he relies less on his hips to produce rotational movement. 
Finally, properly performed shoulder turn helps the golfer to maintain his chest facing the ball as he approaches the ball. This helps him to prepare for his stance and swing path.
How to Properly turn your Shoulders in the Golf Swing
Forming a large turning radius, which is also called shoulder turn, is a component of the golf swing that is sometimes misunderstood.
Recall that although the body pivots around the hips, due to the way the ball is sitting in the stance, the shoulders actually form a larger radius than the hips.
Used properly, large shoulder turn, a.k.a. open clubface out at the top of the backswing, is an extremely useful visual cue for you to follow the ball into the downswing.
However, sometimes golfers can over-do it and create such a large turning radius that their swing gets too wide despite having a narrow clubface angle. This move tends to prevent the club from reaching as low as you need to in the downswing and thus stops the club face from facing the target at impact.
Golfers with an over-large turning radius will typically hit the ball thin where they reached too far over the target line at impact.
So, here is what I recommend. If you find yourself hitting the ball thin, try to purposefully restrict your shoulder turn by saying "keep my shoulders closed".
Try to maintain a nice and small shoulder turn. If you can, try to limit your shoulder turn to an eye's width.
Keep practicing until the clubface is square at impact with no fear of getting the club striking the ball fat.
Shoulder Turn Drills
The most important shot in golf is the 8‛ to 18‛ turn to the hole, known in golf as the shoulder turn. The better you learn and execute this fundamental move, the longer your drives will be.
This on-line drill will help you improve the consistency of your shoulder turn. It is best to work this drill to a distance of 50 to 100 yards and try to execute it every time.
If you cannot make all the shots, work on your long shot percentage first. Use short clubs on the remaining holes until you reach a lower long shot percentage.
Remember, your shoulder turn does not mean raising your right shoulder towards your face. Your shoulder turn means rotating your hips, torso, left shoulder, and left elbow. Feel this rotation as you finish your shoulder turn to guarantee maximum distance. Experimenting and perfecting the shoulder turn goes a long way towards increasing the distance of your drive.
Drill 1: Club Across Chest
This drill helps open up the shoulders and turn the body towards the target. It helps to connect your arms to the chest and chin.
Stand tall and reach the left arm across the chest. Take your right arm down and across the chest.
Drill 2: Shoulder Stretches
Drill 3: Video drill
The goal for this exercise is to get you to properly use your shoulders, and to use them the same way you would when swimming freestyle.
With you hands in the freestyle position, you’re going to make a ìS’ type movement with your shoulders.
At the start of the recovery, you’re going to drop the right shoulder down and turn it over as you pull your hand through.
Then you’re going to drop the left shoulder immediately as you pull through with your left hand.
This is just a single repetition. You’re going to complete 10 of these.