I'm interested in hearing exactly how in this play you are supposed to 1) referee the defenders upper body to know whether it is moving forward or not and 2) judge exactly when one foot touches down making the defender legal.

I'm arguing there are limits to which we can detect parts of a given play. If you want to argue that the defenders big toe touches down before the offensive player initiates contact -- fine. I'm saying I (and any other human) can't see two things at once. Which is why we have judgement. In this play I lean towards calling a foul, whereas you lean the opposite way. That's fine. Just understand that most coaches (who write the rules and to an extent govern an officials advancement) would see this play as an obvious foul.