A platoon system in baseball or American football is a method for substituting players in groups (platoons), to keep complementary players together during playing time.
In baseball, it's usually used to optimize batting performance against pitchers of opposite handedness. Right-handed batters generally perform better against left-handed pitchers and vice versa. Despite some resistance from players who prefer consistent play time, this strategy has been effectively used by managers like Casey Stengel of the New York Yankees to win multiple World Series championships.
In American football, the term "two-platoon system" refers to the practice of using different players on offense, defense, and special teams. Additionally, "platooning quarterbacks" is a tactic where two quarterbacks with different skill sets are used alternately to maximize offensive potential and variability. This requires the defensive team to prepare for two types of quarterbacks, but it's less common in higher echelons of football due to the potential for "quarterback controversies".