In
baseball, a substitute for a runner already on base. Like a
pinch hitter, the pinch runner replaces the original runner in the lineup, so that the original runner is permanently removed from the game.
Because of that rule -- and because there’s a decent chance the runner won’t score no matter how fast he is -- pinch runners are used sparingly. Typically they appear in the late innings of a very close game, when a fairly slow guy gets on base. If the team is desperate for that run to score (e.g. if they’re losing 3-2 in the 9th inning), they might opt for a pinch runner, and the speediest guy available gets the call.
More obviously, pinch runners are used when someone gets hurt but is still on base. Example: a guy gets beaned in the head and taken to the hospital. Somebody’s got to take his place on first base; enter the pinch runner (who, in this case, would probably take over the batter’s defensive position as well).
While you’ll see teams employ pinch-hitting experts, a “pinch runner extraordinaire” is rare. The only one I can think of is the Oakland Athletics’ Herb Washington, hired as a professional pinch runner during the Charlie Finley years. It was apparently a stunt to protest the absurdity of the new designated hitter position.
Source: CBS baseball online library: http://web4.sportsline.com/u/baseball/bol/ballplayers/O/Oakland_Athletics.html