What indicates a substitution is occurring during a game?

A substitution in football is indicated primarily by the action of a player exiting the field. This physical movement is a clear signal to referees, players, and spectators that a player is being replaced, making it the most direct indication of a substitution taking place.

While the referee may blow the whistle to confirm substitutions or a coach may announce them, these actions are secondary to the actual exiting of the player from the field. The player departing shows that the team is indeed making a change, which is essential for maintaining the flow and understanding of the game. Given this context, the focus on the player's exit as the primary indication allows for a straightforward understanding of substitution in the game's dynamics.

The other methods, like referee whistles or coach announcements, support the substitution process but do not serve as the primary action that signifies a change is occurring on the field.