A simple questions. What's the foul to be called?

[GregAlan]

And what is reported to the table? Player A1 has position on the rebound and goes up. Player B1 goes up from behind and initiates contact when reaching for the ball. No "over the back" call, I understand that, but, you (and I) would call a foul on B1 for....what...what foul call do you make here...on the arm? blocking foul?

thanks for the advice...