Under the front -PART 2

[Hawks Coach]

What reference can you provide that says B can move under airborne A after A has left the ground and draw a foul on A? Is there a special rebounding rule that allows B to do things that under any other situation are illegal? Is there an advantage given to B because B didn't try to establish legal guarding position, when clearly in any situation where B is attempting to establish a guarding position B cannot move under an already airborne A?

If you want a layman's term for it, rather than over the back, I see this as B undercutting A. If there is a concept in anyone's head about over the back, it applies to B already being in postion and A violating B's vertical plane AND making illegal contact (technically a push). When B moved under airborne A, B lost any right to the principle of verticality. Any contact is the fault of B, unless I am missing something in the rules here.