A colleague of mine gave me this article yesterday.  I was asking him a question on what I’ve been thinking about the “driver” of rotation at the beginning of the swing, from the anatomical standpoint.  In our facebook discussion last week I said the knee plays NO role in getting the back leg started.  The foot (ankle) advances the body and the femur into the hip begins to turn the body.  The knee is just a connector between the two (a connecting joint).
This article talks about joint function.  The ankle and hip socket are mobility joints (movers) while the knee is a stability joint.  The action many hitting coaches see of the knee moving down and in does happen, but the knee has nothing to do with it.  It is the femur that moves the knee as the body starts the “rotational” move.
http://graycookmovement.com/?p=118