You say to look at frames 2 & 3 for back leg "drive". I see no back leg drive. Check out the angle of the bend in the knee. From frames 2 and 3 there is no change in it. Actually, there is very little change at all from start to finish. The back leg rotates more than it drives. Now I'm not saying there's no push because there is force applied. I'm just proposing that the active mechanism isn't "back leg drive". The back foot rolls over onto the instep, then it contiues to roll over onto the laces. The back knee rotates inward as the centre of gravity moves out. This inward rotation of the back leg happens mostly as the back foot is still fully against the rubber. Man, I wish I could post video!!! I propose that this back knee inward rotation is a huge part of the lower body contribution. More so than "back leg drive".
I have to disagree with this one. Maybe it's just a matter of wording but the centre of gravity is moving forward throughout the pitch until landing. The upper body then comes forward while the shoulders rotate. So, to say the weight is "always back" isn't quite descriptive of how the pitching motion goes. The shoulders may tilt a bit but I believe focussing on it is again "barking up the wrong tree". I propose that the real focus should be on the front hip moving forward and the shoulder tilt will be partially a result of that and partially a matter of style.