Not much is mentioned in Golf Instruction Circles regarding Side bend – particularly in the downswing.
A look at Tour Players clearly show that when they start down, their Right Shoulders actually go down and angled towards the ball. This is also in conjunction with their left hip and Left Shoulder turn out of the way. Ie, the left shoulder will be turning Left and Out of the Way; thus allowing the left side to open up and create Centrifugal Force and Power from the Right Side.
In order to have ample Side Bend in the swing, the muscle chains on the Sides of the Torso need to be limber. If they’re tight, the torso will definitely restricted and prevent the turning of the torso And, also diminish Centrifugal Force (Speed.)
The Windmill stretches will definitely help loosen up your Sides (Obliques), QLO’s, etc.
I personally do Windmills everyday (sometimes mixed in with Triangle Poses).
Read also about Standing Quad Stretch