When an assistant referee observes a foul, what is the correct procedure using the flag?

When an assistant referee observes a foul, the correct procedure is to hold the flag vertically until acknowledged by the referee. This method is important because it communicates to the referee that a foul has occurred while also signaling to the players to maintain the flow of the game. Holding the flag vertically is a clear and distinct signal that draws the attention of the referee, ensuring that the assistant's observation is recognized without causing unnecessary interruption or confusion on the field. This practice helps maintain the integrity of the game and reinforces clear communication between referees and assistant referees.