I had mentioned this topic in a prior thread and was urged to put up a thread on it so here goes a very brief explanation.
There are four major points of resistance a pitchers body needs to achieve in order to transfer energy. Missing any of these four will result in a less than optimal performance. When you look at these pics and listen to the discussion it may sound "mechanical", but in an elite level pitcher these resistance points appear to be very smooth and seamless.....and relatively speaking they are.
Right now I am just posting pics of the Four Points......a discussion on each point will probably create a lot of discussion. I'm a little short of time right now, so I will add in as the discussion flows.
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I think an important key to achieving good "front side resistance" begins with what the drag foot does.
You'll notice in all of Ken's photo examples above that each pitcher's thigh (with knee slightly bent) is oriented forward, roughly right down the "powerline". And shoe laces are also turned in a "forward facing" articulation of the ankle.
I think that this is essential to achieve the correct posture which helps realize good "front side resistance"