backboard
 

Player a1 has possession of the ball under team b basket. Player a1 goes up but player b1 strips the ball out of a1 hands. He strips it so hard that the ball slams against the ground and hits the foam underneath the backboard. Play resumes.
The rule for the backboard mentions the top, the side and the framing but does not mention anything about the bottom of the backboard. Is there a rule for the ball hitting underneath the backboard?

7-1-2

The ball is out of bounds:

a. When it touches or is touched by:

1. A player who is out of bounds.
2. Any other person, the floor, or any object on or outside a boundary.
3. The supports or back of the backboard.
4. The ceiling, overhead equipment or supports.

Bottom of the backboard is in play.

???

(Not doubting you, I just can't recall it right now)

7.1.2 SITUATION A:

The ball strikes the side edge or top edge of the backboard or passes over the top of the backboard and the ball: (a) came from a throw-in from behind the plane of the backboard; or (b) from a pass or try from the front or back of the plane of the backboard. The ball does not touch any supporting brace.

RULING: If a fan-shaped backboard is being used in (a) and in (b), the ball remains live. If a rectangular backboard is used in (a), the ball remains live after touching the side edge, but it is a violation if it passes directly over the backboard. In (b), the ball remains live if it touches a side edge or the top edge if it rebounds and comes down in front of the backboard. The ball becomes dead if it passes over the top of a rectangular backboard regardless of the action which causes it to pass over or whether it comes from the front or back of the plane.

To paraphrase what another member of the forum stated, a few weeks ago, "The backboard has 6 sides - 5 of them are inbounds."

Why was A1 jumping at B1's basket?

that might be how FIBA describes it (the OP is in Ontario, and I forget which rules they use there)

NFHS Rules Book 7-1-2 a. 1. states . . ."The ball is out of bounds when it touches or is touched by the supports or back of the backboard.

Thus, simple logic indicates that the front and all four sides are in-bounds.