Djd, shoulders are not rigidly attached to the rib cage,they are a sliding plate with a ball and socket that can move closer to the spine (adduction), and away from the spine (abduction).
Most cant turn the chest 90 degrees and keep a steady or non moving head, once the chest is rotated as far as it can go, the shoulders then move closer to the spine (rear shoulder), and away from the spine (lead shoulder). This gives the 90 degree shoulder turn.
MizunoJoe, where did you read Austin says to stay on the insides of both feet? I looked through my info and notes and cant find where he says that. Your observation of his left hip over his left foot is correct, from my notes and info this is what he is doing.