Who's Responsible?
 

B1 is guarding A1. Is B1 responsible for contact if A1 gets his head and shoulders past B1's torso?

Straight from the rules book:

10-6-8 . . . When a dribbler, without contact, sufficiently passes an opponent to have head and shoulders in advance of that opponent, the greater responsibility for subsequent contact is on the opponent.

Maybe. It depends on what the contact is. Just because A1 gets his head/shoulders by B1 doesn't give A1 the right to hook B1 or to move laterally into B1. But in general, the foul will be on B1 as, more often than not, it is B1 who is moving laterally into A1.