Is it legal for a defender to slap the ball out of the hands of an inbounder if the ball breaks the imaginary out of bounds line plane? This happened last season when I was refereeing and I was looking for a definitive answer but didn't get one online. My partner and I gave a warning to the defender who slapped the ball out of the inbounder's hands and then gave a technical when she did it again. The coach argued and asked us if it were okay to hit the ball away if the ball broke the plane of the out of bounds line. We said it was never okay but then I read about it later and had second thoughts...(when I reflected about this situation, the point was moot because the inbounder didn't let the ball break the plane; I think the coach was just trying to give us a hard time...)

Even if the inbounder uses the ball to make contact with the defender, it is still a violation by the defender - regardless of whether the ball crosses the plane of the line or not.

Oz is quoting FIBA rules, so ignore his posts with regards to NFHS.

Yes, it's legal to bat, touch or grab the ball if the thrower extends it through the boundary plane.

Thanks so much for the clarification. Now, if ever the freaky thing ever happens in a game, I'll have to take the heat from fans and coaches about the call! Do you ever initiate clarifications to coaches or does the coach have to wait for a time out to ask for one? Should you ever try to clarify a call to an irate coach?

So how does this work? A4 is inbounding the ball, and holds the ball over the plane of the line. Standing inbounds, B5 grabs the ball. Wouldn't this mean that A4 is now standing out of bounds and touching the ball, therefore it is now Team B's inbounds? Is this right?

you could do that
 

or if the swipe was fast enough just let b keep possession -- teach A a lesson

No. A4 is legally OOB. As long as he is holding the ball, he can't be called for causing it to go OOB. If the defender grabs it, it's a held ball. If he knocks it lkoose, it's a matter of where it lands.

You can give a very quick explanation to the coach as long as you're not slowing down the game. If he asks you during a time-out, yeah, give him an explanation. Never explain if no one is asking.

Btw, see NFHS case book play 7.6.3. That's your rules citation.

If defender reaches across OOB line but does not make contact with thrower or ball - warning (2nd occurence is T).

If defender reaches across OOB line and makes contact w/ thrower or ball - T.

If thrower holds ball across OOB line into court, then ball is fair game; steal, jump ball, etc. (no penalty on defender).

See previous rule/case book citation.

T, and constitutes warning, if defender reaches across OOB and contacts ball.

Intentional foul, and constitutes warning, if defender reaches across OOB and contacts thrower.