"The easiest way to put it is, if both players are playing the ball, you have no foul because both players are entitled to the ball. That's important to remember about pass interference with the ball in the air - both players have an equal right to the air. I think there's a misconception that the offense gets more protection, but that's not the case. Defensive players have just as much right.
"If one guy is not playing the ball and one is, and you have that kind of contact, the one not playing the ball will always be called. You can have plenty of contact on a play where a defensive player tries to undercut a route, and he gets shoulder to shoulder, and it's not a foul because he's entitled to that ball."