Ask yourself two questions:
Does B1 have legal guarding position?
Who initiated the contact?
In this case, B1 does have position, provided he is airborne in a vertical manner. Even if B1 goes airborne first, if A1 enters B1's cone
, it's either A1's foul or no foul at all.
We see this kind of thing a lot when one player shot fakes another off their feet. MOST of the time, the defender is off-balance, is not vertical, and winds up landing on the shooter, or pushes him. But the odd time the defender manages to stay vertical - the shooter tries to draw the foul by jumping into the defender, and (hopefully) we pass on the foul.
The problem is, very few fans, coaches, and players understand why there is no defensive foul - so often, all they think is their player got mugged and we didn't call anything.
This falls nicely under my signature....