If you go out of your primairy area of responsability, you interfere with your collegues primairy. That is not necessarily a bad thing, but you have to be very very very cautious to do so. The thing that can happen is that your collegue is about to call the exact opposite of what you as a helping ref are calling. And that is an ugly event you want to avoid. For example this clip , clipped by the great Sam Subby, where two referees want to call the exact opposite. Offcourse this is 3po and not 2po, but it can happen in both systems. How do you prepare to avoid situations like this? Do you talk about it in your pregame?