[Adam]

To elaborate a bit (only a little bit). Let's assume it's an AP throwin.
1. The ball goes directly from the thrower's hand to the rim and gets stuck. Violation on the thrower. B gets the ball and the arrow gets switched to B for the next AP opportunity. Incidentally, if the ball hits the floor first and then gets stuck on the rim, the result is the same.

2. The ball goes from the thrower's hand, is deflected by any of the other nine players, and then gets stuck on the rim. Held Ball, ball goes to B on the arrow, but the arrow will then be set back to A for the next AP opportunity.

3. The ball goes from the thrower's hand and the first time it's touched is a tie up between B1 and A2. Same result as #2.