A scrum is finished once the ball has come out of the scrum.

Once it has, then the opposition scrum-half can tackle their opposite number for the ball.

But in some situations the number eight may dribble with the ball, keeping it in the scrum.

This means the opposing scrum-half cannot get their hands on the ball because it's still in the scrum.

This often happens when the team in possession have an attacking scrum near their opponent's tryline.