What is a backcourt (cross court) violation?

Once a team has the ball in its frontcourt (the half with the ring they're attacking), it can't take the ball back over halfway into its backcourt. It's a violation if a player of that team is the last to touch the ball in the frontcourt and then the same team is first to touch it after it goes into the backcourt.

It isn't a violation if a defender knocks the ball back over halfway.

Signal: blow the whistle, raise an open hand, then wave your arm in front of your body with your index finger pointing to the backcourt. The other team gets a throw-in from the frontcourt sideline near halfway.

In lower Year 3 divisions, backcourt violations are often only called when they're obvious, or not called at all.