[Mark T. DeNucci, Sr.]

NFHS R4-S44 and NCAA R4-S64 define the Principal of Verticality and both rules are identical. While the rule talks in terms of the defense, verticality applies equally to all players on the court. A player who has legally acquired a place on the court is entitled to that place and the space above that space all the way up to the ceiling.
Therefore if B1 places any part of his body into the space directly above A1 and then there is contact between A1 and B1, B1 is responsible for the contact. If B1 and go straight up and reach into A1's space and grab the rebound without any contact between him and A1 there has been no infraction of the rules, but if there is contact even has described in the original play then B1 is responsible for the contact and can be charged with a personal foul. And it should be remembered that if B1 has control of the ball when the contact occurs and you are going to call the foul, it has to be a player control foul on B1.