A1 inbounding on the sideline, passes the ball to A2. The pass never passes through the imaginary plane of the sideline. A2 reaches over the OOB area to catch the pass. Is this legal?

It's a violation, 9-2-3.

Throw-in violation.

Rule 9-2-3

Nope, here is quote from the book--

<b>Rule 7</b>
SECTION 6 THROW-IN ADMINISTRATION
ART. 1 . . . The throw-in starts when the ball is at the disposal of a player of the team entitled to the throw-in. The thrower shall release the ball on a pass directly into the court, except as in 7-5-7, within five seconds after the throw-in starts. The throw-in pass shall touch another player (inbounds or out of bounds) on the court before going out of bounds untouched. <b>The throw-in pass shall not touch a teammate while it is on the out-of-bounds side of the throw-in boundary plane.</b>

Thank You All

Violation in theory, but in practice it had better be REALLY obvious.;)

Also remember that means reaching across the sideline on a throw in on the side or the endline on the endline, but not across the sideline on a throw in from the endline, or vise versa.

Beware of the "hand-off" where A2 reaches over the plane to get the ball from A1.

True, but what about when A1 reaches over the plane to hand it to A2?

That is still a violation because A1 is required to release the pass.