This is an excerpt from Golf Anatomy by Craig DaviesV & ince DiSaia.
When a right-handed golfer initiates the downswing, he shifts his body weight onto his target side (left side) by positioning his target-side knee (left knee) over his target-side foot (left foot). This places the golfer’s lower body into an ideal force-generating position. With the knee over the foot, the quadriceps can function to straighten the knee, and the gluteus maximus and hamstring muscles can contract to create extension of the hip and pelvis. This combined extension movement drives the target foot into the ground. The ground creates a resultant force back into the golfer that can be passed effortlessly through the legs and into the golfer’s pelvis and core. If the pelvis and core are functionally strong and are able to move through the desired range of motion, the force will pass into the shoulder complex. The shoulder complex consists of the muscles connecting the spine and ribs to the shoulder blade and the muscles connecting the shoulder blade to the arm. If the shoulder complex is functioning optimally, this force can be transferred into the arms and, finally, into compression of the golf ball.
In addition, using the legs to position the golfer and create power helps minimize an over-the-top, slice-generating swing. The lateral shift of the lower body onto the target side brings the plane of the downswing forward toward the target. As such, the arc of the club will automatically have a more inside swing path.
When a golfer initiates the golf swing with her upper body, the angular momentum of the golf club forces the club head out away from the body on the downswing. Once initiated, this angular momentum provides resistance through inertia against the golfer’s body, preventing the body from moving forward toward the target. Visually, you see a golfer who appears to have fast hips. It appears her hips are rotating too quickly, which forces the club out and away from the body as the trail shoulder moves forward toward the ball, creating an over-the-top, slice-generating swing plane. Often a player like this is told to slow down the hips. Actually, the problem is not that her hips are turning too fast but that she is using her arms to generate the power and not using her legs to shift forward toward the target. When this player learns to use her legs to push into the ground, her apparently fast-rotating hips will appear to slow automatically, and her club head will begin to attack the ball from the inside more easily.
Players who appear to have fast hips and have trouble attacking the ball from the inside are rotating predominately through the joints in the lower back with minimal rotation actually occurring at the hip joint. This lower-back-centered movement is especially stressful on the spine and supporting muscles. The wear and tear eventually will lead to pain.
Read more about Golf Anatomy.