Shoulder turn is a bit of a controversial topic when you speak to teaching professionals. Some think too much emphasis is put on trying to make as big a rotation with the trunk as possible in order to better synchronize the swing and, more importantly, generate faster clubhead speed. Many teachers feel that if golfers turn back too far, they can lose the spring-like coil needed to swing back down with good energy, or they will increase the risk of injuries—or both. Other teachers point to the coiling techniques of big hitters such as Dustin Johnson and John Daly and argue that the modern golf swing necessitates the need for a healthy turn.
No matter which side of the debate you agree with, Golf Digest fitness advisor Ben Shear says it's important to have good thoracic mobility in order to protect your back from injury, and make a functional swing. You might think of it as shoulder turn, Shear says, but in fact your backswing rotation is propelled by the muscles around the mid-to-upper portion of your spinal column. This is area of the spine is known as the thoracic or "T" spine.
"I don't know too many amateurs who turn too far off the ball," Shear says. "It's a lot more common to see very little trunk rotation. They're basically all arms when they swing."
If you'd like to make a better backswing, start by improving your thoracic rotation by trying the exercise you see demonstrated here by Shear and highly regarded golf instructor Debbie Doniger. Doniger teaches at GlenArbor Golf Club in Bedford, N.Y.