Pitcher Mark Prior of the Chicago Cubs threw a "simulated game" on Tuesday, his second since he injured his elbow last month. His teammate Kerry Wood is also working to recover from a recent injury; Prior has joked that he and Wood are now fine candidates for the "Simulated Hall of Fame." What's a "simulated game"?
It's an informal scrimmage that allows an injured pitcher to test his arm. Simulated games take place at the stadium, with two or three hitters taking turns in live at-bats against a pair of pitchers. (It may happen that both pitchers are recovering from injuries, but most of the time a healthy teammate or coach is recruited for the exercise.) There are rarely any players out in the field, and there's no umpire behind home plate. Either the bullpen catcher or the pitching coach will call balls and strikes and determine what "happens" when one of the hitters puts the ball in play. If it's a hard line drive, they might say it's a "hit"; a weak grounder would be deemed an "out."
When Prior gave up a hit in one of his simulated starts, he had to pitch to the next hitter as if there were someone on base—from the stretch, perhaps. He'd continue to face the same few batters until three "outs" were recorded. At the end of a simulated half-inning, Prior would head to the dugout and wait until the other pitcher recorded three outs.
Simulated games rarely last for more than three or four innings. Sometimes stats are kept on the simulated runs and hits, but the more important figure is how many pitches were thrown. An injured player who makes a good showing in a simulated game and demonstrates that his arm has recovered might then be sent to the minor leagues for a tuneup start, or he might return to the team right away.
Some pitching coaches and trainers use recorded stadium sounds to make the simulated games more realistic. For a recent simulated start, Cleveland's Kevin Millwood faced fellow Indians in the replica jerseys of an opposing team—the hitters even imitated the batting stances of the players whose names they wore.
In general, players will swing away during simulated at-bats, but in Prior's game on Tuesday, the designated hitters were asked to test his fielding with a few bunts. Hitters tend not to take these plate appearances too seriously, but in one documented case, an infielder with the Cleveland Indians threw his helmet onto the field in frustration—after getting called out on simulated strikes. The "umpire" who called him out was the team's head groundskeeper.
Explainer thanks Will Carroll of Baseball Prospectus, John Zajc of the Society for American Baseball Research, and reader Bradley Johannsen for asking the question.