When signaling a backrow attack, what action is being indicated by the referee?

When signaling a backrow attack, the referee is indicating that a back row player has violated a rule. In volleyball, a back row attack violation occurs when a player in the back row attempts to attack the ball from a position that is outside of the specified area, specifically in front of the attack line. This can mean that the player has jumped from in front of the attack line to hit the ball, which is not allowed unless they are behind the line when the ball is hit. By signaling this violation, the referee communicates to players and spectators that the play is not valid due to the rule being broken, ensuring the fair application of the game's regulations.