Offensive Player Initiates Contact on Jump Shot

[tlynch]

If an offensive and defensive player are running parallel to each other down the floor on a fast break, and the offensive player lowers his shoulder and pushes the defensive player off of his path - who is the foul on?

Lets take an extreme example, a defensive player has a running start and jumps from the foul line to block a shot of offensive player A1. Player A2, who was stationary when B1 left the floor, sees B1 in the air and steps into his flight path in the middle of the key and gets hit. Is that a foul on B1?

Anytime a defensive player leaves the floor and is not vertical an offensive player can simply step in his way and if there is contact it is a foul on the defensive player?