Defender intentionally falls onto hands and knees...

[Adam]

My first thought is an (leaving aside the valid point regarding the apparently missed PC calls) intentional foul. B1 is intentionally tripping A1.

That said, I understand the point about the rule not really having a provision for a foul here. B1 is stationary and not holding an illegal position. My only thought is the "purpose and intent" clause, not allowing a player to gain an unfair advantage not intended by the rules. I can't help but think of this move as an unfair tactic gaining an unfair advantage by tripping a "blind" opponent.

I'm still not sure my initial thought is correct.