Watching between players
When two players challenge for the ball, as a referee, we must be able to observe the space between the players. This allows us to see if the challenge is fair. If one player pushes, trips, undercuts, or holds their opponent, we can observe it well. But if the space is blocked by one of the players, we are not able to properly observe the challenge.
In order to watch the space between two players, we must use not only our agility and stamina to keep moving but also anticipation and focus. Here is a good example of a well-focused referee anticipating the next play while using his agility and stamina.