You're making up rules. Of course the defense is trying to prevent the offense from completing a throw-in -- no rule prohibits that.

by B1 grabbing the ball and preventing A1 from completing the throw-in while A1 still has the ball is not a violation?

mbyron, I also asked that be shown the rule &/or case book play that says otherwise, which you did not do.

How about 9-2-10?

Read the OP again. A1 extended the ball over the line, iow, out over the inbounds area.

Now tell us how 9-2-10 applies.

There was never an explanation if A1 pulled the ball back over the plane before the whistle.

If the ball did recross the plane then B1 would be in violation?

ART. 10. . . The opponent(s) of the thrower shall not have any part of his/her person through the inbounds side of the throw-in boundary-line plane until the ball has been released on a throw-in pass.
NOTE: The thrower may penetrate the plane provided he/she does not touch the inbounds area before the ball is released on the throw-in pass. The opponent in this situation may legally touch or grasp the ball.

A1 holds the ball across the plane, true, however what constitutes the legality of B1 being able to grab the ball causing the "jump ball"?

Also why didn't A1 just release the ball as soon as B1 grabbed it?

In this sitch, there are many what ifs or whys.

I believe A1 causes the ball to be out of bounds, and therefore not a jump ball. 7-2-2. IMO.

Case book play 7.6.4 Situation F.

I like it when the answer is so obvious that all I have to do is post a citation that addresses it exactly! (And I've been able to do that twice just in this thread!)

Bzzzzzt. Thanks for playing. ;)

The rule is pretty simple. If A1 holds the ball on the inbound side of the plane, Team B is free to play the ball. That means he can knock it out of his hands/cause a held ball. If Team A doesn't want this to happen, they best stay on the out of bounds side of the plane. In this sitch, if A1 were to hold the ball on the inbounds side of the plane and just dropped it whenever B1 grabbed it, the ball would be in play. A1 just basically gave the ball to Team B

How does a thrower cause the ball to be out of bounds whenever he is legally out of bounds and the throw-in hasn't ended? The correct call is a held ball.

The throw-in did not end. A1 is legally out of bounds.

Well, that certainly addresses the OP with an answer....I'm still wondering why player control or team control comes into the picture at all. If two players hold the ball, who's controlling it doesn't seem to be part of the question at that time, if one player is out of bounds. The player OB makes the ball OB, regardless of team/player control, doesn't he? Again, I can read 7.6.4 F, but just curious about why control is part of the debate, and why OB doesn't supercede here. I appreciate your response...it seems dead on point.

Yes it really is obvious, I never thought of it like stated.

7.6.4 SITUATION F: Thrower A1 inadvertently holds the ball through the endline plane during a throw-in. B1 is able to get his/her hands on the ball and A1 cannot pull it back.
RULING: There is no player or team control during a throw in,
therefore a held ball is called, resulting in an alternating-possession throw-in. If the original throw-in is an alternating-possession throw-in, Team A still has the arrow following the held ball.

How can a ball be OOB if it was not inbounds via player/team control to begin with?

The throw-in never ended with the ball in control inbounds.

The clock should never have started as there was no player/team control inbounds.

Also case book play 6.4.5SitB.

Tsk, tsk, tsk....

Wrong again. I'd tell you to stick to timing but it looks like you don't really know what you're doing in that area either.

Player or team control has got diddly-squat to do with starting the clock on a throw-in. The pertinent rule....5-9-4... has already been cited.