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The term derives from baseball. A pinch hitter is a player who replaces another player during the game, comparable to the substitute players in football/soccer.
In gift exchanges, each participant is due a fanwork from another participant. If a participant is not able to finish their assignment and submit a completed fanwork, the moderators will call for pinch-hitters to ensure that every participant receives something. Some mods call for pinch-hitters to volunteer at the beginning of a challenge so that they have a group to send pinch-hit requests to at the time-crunch end of the challenge, when most dropouts occur.
Pinch-hitters are generally not required to be, or forbidden from being, participants in the challenge.
There is a livejournal community, fic_pinch_hit, where mods who need an emergency pinch-hitter can post a request. It was founded by fanofall on Nov. 2nd, 2007, and is an extremely low-traffic community.