Relief Pitcher
Everybody who pitches after the starter, and between them they now throw four innings of every game.
What it actually is. A relief pitcher comes into a game already in progress. The roles have names: the long man covers multiple innings when a start collapses, middle relievers work the fifth through seventh, the setup man usually takes the eighth, and the closer takes the ninth with a lead. Under the three batter minimum rule, a reliever must face at least three hitters or finish the inning.
What somebody who knows the game gets out of it. Relievers are better than starters pitch for pitch and always have been, because they throw one inning at full effort with their two best pitches and never see a lineup twice. That is the whole edge. It also makes them volatile: sixty innings is a small enough sample that a good reliever and a bad one can post the same ERA in a season, which is why clubs pay for strikeout and walk rates rather than for ERA and why the reliever market is the most efficient part of a roster to churn.
The numbers worth knowing. A club carries eight or nine relievers and gets roughly 550 to 600 innings from them. A reliever throwing 70 innings has had a heavy year. Leverage index is the number that separates the man pitching in the eighth of a one run game from the man pitching in a blowout.
What people get wrong about it. The three batter minimum killed the one out left handed specializt as a job, and that was the point of the rule. It was introduced to shorten games by cutting pitching changes, and it changed roster construction more than it changed game length.
At Little League and travel ball. There is no such thing as a reliever at youth level, only the next pitcher, and the decision is almost always a pitch count decision rather than a matchup one. The useful youth version of leverage is simple: use your best available arm in the inning where the game is actually being decided, not automatically in the last one.
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