As we know there is an exception for a player blocking in the back when a player "turns his back to a potential blocker who has committed himself in intent and direction or movement".
Consider this situation. The runner has crossed the line of scrimmage and lead blocker A20 lines up and is committed to block B40 from the front and below the waist 10-2, but just before contact B40 had rotated and A20 ends up hitting him below the waist in the side outside of 10-2. Your thoughts?