Umpires check pitchers' hands primarily to ensure they are not using illegal foreign substances, such as sticky stuff (e.g., spider tack, pine tar, or banned adhesives), that can give them an unfair advantage by enhancing grip and increasing the spin rate on the baseball. This inspection is part of maintaining the integrity and fairness of the game. Umpires typically perform these checks during pitching changes or between innings by thoroughly inspecting the pitchers' hands, looking for any unusual substances like clumps or discoloration. The goal is to deter and catch pitchers applying substances that are against the rules, which could otherwise affect the pitch's behavior and impact the outcome of the game. The checks have become routine and are enforced with ejections and suspensions for violations.
This process is done after innings rather than before because pitchers often apply these substances during the game itself, and the post-inning checks help umpires catch evidence of such use. The inspections focus on the hands rather than other equipment in recent years to streamline enforcement but may include gloves, belts, or hats occasionally.
