Pinch Runner
A substitute sent in to run for somebody already on base, and once he goes in the man he replaced is done.
What it actually is. A pinch runner replaces a runner already on base. The replaced player is out of the game and cannot return, which is the whole cost of the move. The pinch runner takes that spot in the batting order and normally has to be given a defensive position or be replaced again the next inning.
What somebody who knows the game gets out of it. A pinch runner is one of the most expensive small moves a manager can make, because it burns a bench spot, removes a hitter for the rest of the night, and buys perhaps a fifteen percent better chance of scoring one specific runner. That is why it is almost exclusively a late innings, tie game, best hitter already on decision. Clubs with three man benches make it far less often than clubs in the era of five man benches did, which is a roster construction fact rather than a strategic one.
The numbers worth knowing. A stolen base moves run expectancy by roughly .2 runs when it works and costs roughly .45 when it does not. That asymmetry is why a pinch runner has to be a better runner to be worth the bench spot.
What people get wrong about it. Courtesy runner and pinch runner are not the same thing and the difference matters enormously below the professional level. A courtesy runner runs for the pitcher or catcher to save time and the player he ran for stays in the game; a pinch runner is a substitution that ends somebody's night.
At Little League and travel ball. Youth rules almost always allow a courtesy runner for the catcher, and often for the pitcher, specifically so the catcher can put his gear on. Whether it is required, who is eligible to be the runner, and whether it must be the last batted out varies by sanction, which is one of the four rulebooks comparisons on this site.
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