The backswing is when the body begins to move the club. The backswing is the portion of the swing that places the body in the correct position to begin the downswing. During the entire backswing the body begins the recruitment of energy that will be transitioned at the top of the backswing towards the ball.
Key points from a biomechanical analysis of the backswing are: as the club moves backwards shear force is applied to the anterior portion of the right foot while at the same time a posterior shear force is applied to the left foot. (Fleisig, Biomechanics of Golf) This is the beginning of torque development in the body that will be transitioned into the clubhead at impact.
Rotation of the knees, hips, spine, and shoulders continues during the backswing. The order of this rotation is the knees, hips, and torso occurring around an approximate vertical axis through the center of the body. This creates additional torque to be translated into the clubhead in later stages of the swing. The important point to remember in the backswing is that the entire rotation of these body parts occurs around an imaginary axis of the body. The body during this portion of the swing is creating/storing energy to be released during the downswing phase of the golf swing.
The biomechanical analysis of the backswing indicates this is the stage of the swing at which speed development begins. The process by which this occurs is through the creation of torque and the development of torque by the body requires rotation. Rotation in the lower body, torso, and shoulders is contingent upon a number of physical parameters such as joint range of motion, muscular extensibility, stretch reflex, and segmental strength in the lower body, hips, core, and upper back. If any of these physical entities are lacking, the ability to execute the backswing and develop torque will be diminished.