You're Correct!

Many amateurs fall into the trap of trying to use as much force as possible by swinging the golf club with their arms, however, the majority of the swings power comes from the core strength one has.

A good and efficient back swing uses the bodies core strength, combining the rotation of the shoulders with a strong 90 degree wrist hinge during the swing. The rotation on the back swing is an important component and does not need to be done at a fast pace. The back swing needs to be meticulously built but maintain some fluidity.

A correct shoulder turn is a great way to build power during the back swing and this can be recognised by a 90 degree shoulder turn and good checkpoints for this are as follows:

Your back pointing to the target
Your chest pointing backwards
Your left shoulder under your chin

The club does not necessarily need to be parallel to the ground at the top of swing because a players rotation may not allow for this so this doesnt mean that the left arm needs to break to get the club parallel. A straight left arm combined with 90 degrees of shoulder rotation and wrist hinge will give a player a large build up of power and energy at the top of swing, like a coil that has been twisted to its maximum and when you release the coil its tension unwinds fast and powerfully.

The most effective method of practising this move is to position yourself side on to a mirror and rotate your shoulders. Look up to see how much you have rotated. Hopefully you have your chest pointing back to the mirror and you have maintained your posture at the same time.

The golf club can reach the horizontal point at the top of swing with a bend in the left arm or wrists. This does not mean the golfer now has a full, powerful back swing. If anything, due to the lack of body rotation and the addition of levers, the swing has actually become less powerful and efficient. Once the left arm bends at the top of the swing all tension is lost and the down swing is spent re-straightening this arm towards the ball rather than uncoiling all the built up energy into impact. Golf swings can reach the parallel point at the top of the back swing with just 30-40 degrees of shoulder rotation which reduces the amount of coil and recoil potential. Ask yourself this question… which part of my body has the biggest and most powerful muscles?

1) My arms?
Or
2) My chest, back, sides, gluteus, thighs, shoulders and stomach combined?

Hopefully you agree its answer number two!

The key to a good shoulder rotation is stability. The spine angle must remain at the starting angle throughout the swing with the shoulders rotating around it. If the body starts to arc or move in a forward direction towards the target during the swing, the player is more than likely reverse pivoting. This will have a profound effect on the players consistency of strike. Use a mirror to make sure you are not laterally moving side to side whilst trying to make the correct rotation. Practice moving the opposite way at the start to counteract your initial movement.

Shoulder rotation needs to be stable and must maintain the strong fundamentals that were evident at the start of the golf swing. When a full 90 degree rotation is attempted or successfully achieved, make sure that you have maintained your posture. If your shoulders are parallel to the ground at the top of the swing, you have straightened out your posture in order to achieve the shoulder turn. If you are not capable of the turn, rotate as much as you can whilst maintaining your starting posture. A good swing can still be achieved.

Before you try to correctly rotate your body during the swing it may be best to check your body is capable of the movements. There are many flexibility tests that can be done to check your range of movement as well as lots of stretches to improve your range of movement.