Break-away dunk - off the backboard. Legal?
 

Player A1 makes a steal at mid-court and goes in for an uncontested score. As he approaches the basket, he throws the ball off the backboard and dunks it. Legal? After he grabs the ball following his last dribble isn't throwing it off the backboard considered another dribble and therefore this should be ruled a double dribble? I've seen this play a few times and I don't think I've ever seen a violation called.

Legal. off your own backboard, I consider it a shot.

IT's only a dribble to throw it off your opponent's backboard.

there is (or was) a specific case in both NFHS and NCAA where this is legal.

Dribble rule
 

*9.5 SITUATION: A1 dribbles and comes to a stop after which he/she throws the ball against: (a) his/her own backboard; (b) the opponent’s backboard; or (c) an official and catches the ball after each. RULING: Legal in (a); a team’s own backboard is considered part of that team’s “equipment” and may be used. In (b) and (c), A1 has violated; throwing the ball against an opponent’s backboard or an official constitutes another dribble, provided A1 is first to touch the ball after it strikes the official or the board. (4-4-5; 4-15-2; Fundamental 19)

I would also say legal in NFHS...

I was about to say illegal now in NCAA with the rule change this year about the ball being considered in downward flight after it touches the backboard...but you could argue that the player throwing it against the backboard isn't a legal try-for-goal, in which case it's legal there also.

Modified sitch
 

Does case play 9.5 (a) imply that player A1 who has ended his dribble can throw the ball against his backboard (clearly not a shot attempt), retrieve the ball and dribble again?

rfp,
One of the challenges you face here is how can you tell for sure? I have seen some unbelievably ugly shots over the years. To keep it simple, if a player puts something in the direction of his own basket, consider it a shot.

What about an alley-oop pass? If it's a shot, then the player who catches and dunks has committed an offensive goaltending violation.

If it hit the backboard and was on the way down, could be offensive GT.

But for GT, the ball must have a chance to go in. For a "self-pass" off the bb to go in, wouldn't the ball need to be in the cylinder, for the chance to go in provision to be possible. Because, once the ball is past the cylinder, it no longer has a chance to go in.

We don't know where the ball hit the backboard. If it was on its way down and still has a chance to go in [as you pointed out] I don't think it needs to be 'in the cylinder'.