From BR Bullpen
A defensive shift is when the fielders move from their normal positions for some tactical reason. The most common shifts are used in response to specific game situations, such as a runner on base, and are seen in almost every game. Less common shifts are a response to strong batting tendencies of an individual hitters. On some very rare occasions, a team may actually move a fielder from the outfield to the infield or vice versa.
 Normal Positioning
Defensive players normally move around a bit within their normal positions. Infielders move closer to the plate against fast batters and further from the plate against slower runners. Outfielders move closer to the plate against hitters with little power and further back against ones with more power. Both infielders and outfielders will move slightly to one side or the other if the batter is known to hit more balls to one side of the field than the other. Infielders will also shift depending on the pitch selection, though those shifts are delayed until after the pitcher starts his motion so that they won't give the batter information about what pitch to expect.
 Situational Shifts
The most common defensive shifts are used in response to specific game situations. These movements are common enough that they aren't always viewed as shifts.
- Holding a runner. When a runner reaches first base, the first baseman will stand with one foot touching first base so that he can receive a pickoff throw from the pitcher, rather than his normal position a few steps away from the foul line. With a runner on second base, the shortstop or second baseman may move closer to second to receive a pickoff throw or convince the runner that a pickoff throw is possible. The fielder in that case won't move all the way to the base as the first baseman does until he must move to receive the throw. Because the fielder plays behind and out of the view of the runner when the runner is watching the batter, the fielder can move back to his normal position as soon as the pitcher has committed to the pitch.
- Double play position. With a runner on first base and fewer than two out, the middle infielders position themselves closer than normal to second base so that they can quickly reach the base to serve as a pivot man on a double play.
- Infield in. With fewer than two out and a runner on third base who represents an important run, the infielders will move closer than normal to the plate. This increases their chances of making a successful throw to retire the runner if he tries to score, but increases the chance of a ground ball making it past the fielders.
- Infield half way. If the defense might want to play the infield in but could also escape from the scoring chance with a double play, they may play a hybrid position in which the corner infielders play in while the middle infielders play in their normal double play positions.
- Bunt defense. If the defense expects the batter to attempt a sacrifice bunt, one or both corner infielder may position himself very close to the plate in order to field the bunt as quickly as possible and either retire the lead runner or turn a double play. The third baseman will play in if the runner is at first base and the first baseman must stay back to hold the runner, and the first baseman will play in if the runner is at second base and the third baseman must play back to receive a throw. Such aggressive positioning is only used if the defense is very sure that the batter will bunt, because the corner infielder will be hopelessly out of position at at increased risk of injury if the batter swings away.
- Shading the lines. When protecting a small lead late in the game, the first and third basemen will move closer to the foul lines when the bases are empty. This is supposed to cut down on the number of ground ball doubles hit down the foul lines and thus decrease the chance of a runner reaching scoring position.
- Shallow outfield. With the winning run at third base and fewer than two outs, the outfielders will play very shallow so that they can throw the runner out at the plate on any ball they can catch. They concede a hit on some balls that they could normally catch for an out because a sacrifice fly is just as bad as a hit.
 Batter Shifts
Although fielders will change their position slightly for every batter, those shifts are normally small, and the fielders start play within a few steps of the center of their normal zone on the field. Some batters have such strong batting tendencies that teams will use an extreme shift against them. In an extreme shift, fielders will move so far from their normal positions that some of them will start play in a position normally occupied by another fielder.
Such extreme shifts are almost always used against power hitters with very strong tendencies to pull the ball. The most famous example of a shift was that used against Ted Williams. In Williams's words, in the most extreme version of shift:
- The third baseman, Kenny Keltner, moved behind the bag at second; the shortstop, Boudreau, moved to the right of second base; the center fielder, Pat Seerey, moved into the right fielder's position; the right fielder moved to the line; the second baseman moved closer to first and back on the grass in short right; the first baseman moved to the line behind first. The only man remaining to cover the entire area left of second base was the left fielder, George Case, and he was about thirty feet behind the skin of the infield.
Although the shift against Ted Williams was most famous, it was not the first extreme shift. Teams had used extreme shifts against both Ken and Cy Williams, and such strong shifts are sometimes called "Williams shifts" without explicit mention of which Williams. Babe Ruth also faced a defensive shift, though only in the outfield. Contemporary players who often face a defensive shift include Barry Bonds, David Ortiz, Jason Giambi, and Gary Sheffield. Shifts against right handed batters like Sheffield can't be quite as extreme as those used against left handed batters because the first baseman needs to remain close to first to take throws from the other infielders if the batter hits a ground ball. Teams usually do not use shifts that are as radical when there are runners on base, and when they do, it will sometimes result in one runner stealing two bases on the same play.
Batter shifts were relatively exception until the 2010s, when they began to be used regularly by manager Joe Maddon of the Tampa Bay Rays, and not just against extreme pull hitters. The availability of precise data on the hitting tendencies of each batter made it possible to design a custom defensive positioning for each batter, resulting in many more instances of a defensive shift being used in a typical game, especially as a number of other teams began to imitate the Rays' approach.
 Exotic Shifts
It's also possible, though rare, for teams to use five man infields or four man outfields. Those defenses are used when the batter is known to have extremely strong ground ball or fly ball tendencies. A five man infield is also sometimes used as a defense in a situation that demands a bunt. Another instance in which a five man infield is sometimes seen is in the bottom of the ninth or greater inning when the batting team has a runner on third base and less than two outs. In this case, since almost any ball hit out of the infield - whether a hit or a sacrifice fly - will score the winning run, the defensive team may bring a man from the outfield to fill a gap in the infield. In any case, such exotic defenses are restricted to critical situations, and are rare enough to be memorable years later.
The most recent memorable 5-man infield used was in Game 3 of the 2008 World Series, when Eric Bruntlett of the Philadelphia Phillies reached 3rd base with no outs in a tie game in the bottom of the 9th inning. The Tampa Bay Rays walked the bases loaded and brought in Ben Zobrist from right field to play in the infield. In the end, the strategy did not work, as a slow roller by Carlos Ruiz toward third base allowed Bruntlett to score the winning run.
Combining the two types of shifts, the Los Angeles Dodgers used an extreme shift in the 12th inning of a game against the San Diego Padres on August 29, 2014. With pull hitter Seth Smith at the plate and the bases loaded with one out in a tie game, the Dodgers not only brought in OF Andre Ethier to be a fifth infielder, but then lined up four of these infielders in a straight line between first and second base, with the third baseman at the normal shortstop position, for an alignment never before seen in a game. Smith responded by hitting a ground ball towards the congregation of players, which was fielded by 2B Alexi Amarista, who threw out the runner at home; the Dodgers were unable to complete a double play, however, and the next batter, Yasmani Grandal, hit a game-winning single.