Peter claimed that Kaymer's lower back angle becomes steeper at impact because of his straightening left leg which makes his left hip then higher at impact then it was at address.
I pointed out that the same steepening is seen in Mickelson and most every other great golfer even when the leading leg is not straightening. Also if all things remain equal and just the lead hip rises I would expect a flatter back, not steeper.
Anyway I think it is obvious that a straightening leg is not the cause. I am however open to any evidence that would show otherwise. As of yet none has been presented.