From the POE's:

2. Team control, throw-in. The relevance of team control during a throw-in only applies when a member of the throw- in team fouls. Such fouls shall be ruled team control fouls. Team control during a throw-in is NOT intended to be the same as player control/team control inbounds. Team control inbounds is established when a player from either team who has inbound status gains control of the ball. During the throw-in, 10-seconds, 3-seconds, frontcourt status, backcourt status, closely guarded, etc., are NOT factors as there has yet to be player control/team control obtained inbounds.
With specific regard to the backcourt violation; a team may not be the last to touch a live ball in the front court and then be the first to touch a live ball in the backcourt, provided that team has establish player control/team control on the playing court (either in the backcourt or frontcourt). BY RULE EXCEPTION, during a throw-in a team may leave the front court, establish player control/team control while airborne and land in the backcourt. This is a legal play and ONLY applies to the first player of the offense who touches the ball PRIOR to the end of the throw-in.

This only applies to an Offensive player? A defensive player (B1) cannot jump from B's frontcourt, catch a throw-in in mid-air, then land in B's backcourt?

The throw in ends when one team gains possession, correct? That would make (old) defense (B1) (new) offense, correct?

Edit- I'm an idiot, sorry. Just ignore this response.

And the throw-in ends when the ball is legally touched -- so the excpetion ends then, too.