Major League managers maneuver against each other during games. Each has a roster of 25 players with different skills and strengths, and it is their job to put the players into a position to succeed. They can move players around the field, but once a player comes out of the game, he can't return. Substitutions can be made any time the ball is not in play, including between pitches.
A manager may choose to insert a player with superior defensive skills, usually when the team has a lead late in the game. The defensive substitute takes the place of the exiting player in the batting order but does not necessarily need to take over that player's position. For example, a manager wishing to replace his right fielder may choose to move his current center fielder there, and bring in a new player to take over in center. That substitute would replace the old right fielder in the lineup.
A manager can pinch hit any player who has not yet entered the game. This can occur even in the middle of an at-bat. Such a pinch-hitter inherits the count, if any, and the exiting player's spot in the lineup. The manager can also double-down, pinch hitting for the pinch hitter before the at-bat takes place. This may happen if the opposing manager changes pitchers in reaction to the first substitute entering the game. Even though the originally-announced pinch hitter never actually played, he is ineligible to come back into the game. A manager can also insert a pinch runner for any player occupying a base, with that player taking the lineup spot of whoever he replaced.
Go to the Bullpen
Managers may substitute a pitcher with a player who has not yet entered the game, even in the middle of an at-bat, as long as the exiting pitcher has faced at least one batter. This requires foresight, as relief pitchers take a few minutes to warm up in the bullpen. It also requires strategy, since each new pitcher must face at least one batter. Managers can use a mound visit to stall the game, giving a relief pitcher time to warm up. The coaching staff is allowed one timeout per inning per pitcher, where a coach can visit with the infield defense on the mound. A second mound visit for the same pitcher during an inning results in the pitcher's automatic removal.
The Double Switch
American League lineups feature a 10th player, a designated hitter who bats for the pitcher and does not play defense. National League rules do not employ the DH. Pitchers generally hit poorly, giving the opposing team a near automatic out. But a manager can use the double switch to keep the pitcher from getting to the plate too soon. To do this, the manager makes a defensive substitution at the same time he brings in the new pitcher, and simply switches their spots in the lineup. For example, if the pitcher's spot is due to lead off the next inning, while the second baseman would bat ninth, the manager may replace both players at the same time. He could then insert the new second baseman in the pitcher's spot in the batting order, while the pitcher would take over the spot that had been occupied by the previous second baseman.