One of the biggest questions that fans new to baseball ask is why there is a parade of relief pitchers in the late innings of a game, especially in close games.
It’s not because the relief pitcher – who might have thrown to only a batter or two – is tired. It’s because the manager is going by the book.
It's all about the breaking ball
It’s considered common knowledge in baseball that it’s harder for a left-handed hitter to hit against a left-handed pitcher, and it’s harder for a right-handed hitter to hit against a right-handed pitcher.
The reason is physics. A curveball or slider thrown by a left-handed pitcher will break down and away from a left-handed hitter, and vice-versa. This makes it harder to hit.
The theory is proved by statistics. For example, take Chicago White Sox slugger Jim Thome, one of the best left-handed hitters in baseball. In 2006, Thome batted .288 with 42 home runs in 490 at-bats.
Thome had 299 at-bats against right-handed pitchers and 191 at-bats against left-handers. He batted .321 with 36 home runs against right-handers, and .236 with six home runs against left-handers.
Broken down further, Thome hit one home run every 8.2 at-bats against right-handed pitchers, and one home run every 31.8 at-bats against left-handers.
Conversely, left-handed pitchers fare much better against left-handed hitters. J.C. Romero is a journeyman left-handed relief pitcher who now plays for the Boston Red Sox. In 2006, when playing for the Los Angeles Angels, left-handers batted .202 against him. Right-handers hit .382.
Managers try to counter this by using pinch hitters. Teams like to keep a balanced bench so they can use a right-handed hitter to pinch hit against a left-handed pitcher, or a left-handed hitter to pinch hit against a right-handed pitcher.
Sometimes, the hitter called on to pinch hit might not even face the pitcher. For example, let’s say Romero is on the mound and Thome is due up. Because it would be a poor matchup, Thome’s manager calls for a right-handed player to pinch hit. But after he’s announced, Romero’s manager decides to call a right-handed pitcher from the bullpen. To counter that, the White Sox manager could call for a left-handed pinch hitter (he can’t use Thome again after he’s been substituted for).
Thankfully for all involved, the substituting couldn’t go on infinitely. There are only so many pitchers in the bullpen and so many hitters on the bench.