The role of the right knee in the backswing is very simple, but incredibly important. The importance of the flex in the right knee is undeniable. The flex in the right knee helps keep the hips more level in the backswing. If the right knee straightens, it pushes the right hip up and consequently the left hip down resulting in too much left-hip tilt in the backswing.
Where there is too much hip tilt to the left in the backswing, it’s difficult to shift your weight properly to the right in the backswing. Therefore, the player inadvertently creates a reverse pivot, or reverse weight shift with too much weight on the left foot at the top of the swing.
Another key reason to keep the right knee flexed is how it is used in your transition into your downswing. Your right knee is a key source of driving power of your lower body in the downswing. It’s very difficult to drive off a straightened right leg in the transition. Therefore, the upper body tends to dominate the downswing resulting in poor swing plane and loss of power.