What must a coach do when making a substitution for a player during an at-bat?

When making a substitution during an at-bat, the correct action for a coach is to call time with the umpire before proceeding with the substitution. This process is essential because it ensures that the game is being conducted in accordance with the rules. Calling time allows the umpire to officially recognize the substitution and make the necessary changes in the scorebook, ensuring that the game maintains proper protocol and order.

Notifying the umpire is crucial in these situations to avoid any potential confusion or disruption to the flow of the game. It also helps maintain the integrity of the pitching and batting orders, which can affect strategy and outcomes during the game. Thus, properly engaging with the umpire is a fundamental part of following the rules surrounding substitutions.