It is well understood that the movement of the hips and lower body are an important part of swing movement, rotation and speed, one factor that is misunderstood is the importance of the stability of the lower hips as you move throughout the swing.
In essence, the lower body is a support system for the upper body and in many ways, the lower body is the braking system for the engine of the upper body. So how exactly does the lower body both move and support the upper body? It’s all in the knees.
As you rotate into the backswing, the trail knee braces the leg so the pelvis can turn. But keeping the knee still is critical as it limits the amount of hip rotation. If the one straightens it can cause an over-rotation and too much pelvis movement.
Once you have reached the top of the backswing with the trial knee stable and hips rotated, the stable trail leg moves your pelvis into the flexed lead knee.
Once your lead knee is flexed, your lead side has put on the brakes. You must keep the lead knee flexed and in position throughout the rest of the through-swing and into your release. Keep in mind that as your lead knee is flexed your pelvis can continue to rotate into the through-swing.
As you continue to rotate through the ball into the finish with the lead knee flexed and trail foot is down, another limit has been created in the movement of the pelvis. Weight had moved into the lead foot and with the trail foot down and the pelvis limited, the upper body can separate from the lower body and produce maximum amounts of speed. By keeping the lead knee in position and the trail foot down, you have created the braking system for the lower body so that the upper body can generate power.
The rotation of the pelvis, while only 40 degrees rotated closed in the backswing and 35 degrees open at impact (at total of 75 degrees) tends to be one of the most difficult movements in the swing as we age therefore it must be something that you continue to develop and maintain as you get older. (I recommend showing the above photo to your personal trainer and mention that you need to be able to achieve this position). It will also take stress off of your back.
To find out more about the Single Plane Swing in 3D, visit Singleplane3D.com.