Defensive Positioning
Putting fielders where this hitter usually hits it.
How it actually works. The alignment starts from a hitter's spray chart and is adjusted by who is pitching and what he throws, because a hitter's contact point moves with the pitch. Against a hard sinker inside he pulls more; against a changeup away he goes the other way. Count matters too, since a hitter behind in the count shortens up and uses more of the field.
Why it is done, and when it is wrong. The important idea underneath is that a defense is not standing where balls are hit on average, it is standing where the most balls it can actually convert into outs will be. Those are different, and it is why the extreme alignments looked so strange. The professional game now restricts how far the infield may be moved, with two infielders required on each side of second base and both feet on the dirt, which pushed the same thinking into subtler shading rather than removing it.
What people get wrong about it. Alignment is a probability bet and it will look stupid several times a game. A ground ball through a vacated hole is not evidence the shift was wrong, in the same way a successful bunt is not evidence the defense should not have played back.
At Little League and travel ball. Youth spray tendencies are enormous and almost nobody uses them. At ages where most hitters cannot get around on velocity, simply moving the outfield toward the opposite field against a young pull hitter is worth several outs a tournament.
Batter's Eye: look up anything in baseball