All those years of hearing that it’s good to maintain some right knee flex (for right-handers) during the backswing? Forget about it.
Extending the back leg (losing flex) and flexing the left knee forward during the backswing is a necessary motion to make an effective and powerful backswing pivot. It allows the hips to turn more, which allows the shoulders to also turn more. Both the hips and the shoulders turn on an inclined plane, as the left shoulder turns down and in and under the chin and the left knee and hip flex forward and downward. This allows the right hip to go back and up on an inclined plane and creates greater hip rotation and depth going back, which creates the potential for more acceleration and power coming down. This is a function of the back-leg losing flex, or extending, and the left knee flexing forward.
Remember, the goal is to maximize the body rotation (pivot) on the backswing to set the stage for a faster rotation on the downswing. This 3-D motion defines the golf swing.