Bringing the Infield in
Infielders move in to cut down the run at the plate.
How it actually works. The corners come in level with or ahead of the mound and the middle infielders come to the edge of the grass. From there a routine ground ball becomes a throw to the catcher instead of a throw to first. The cost is enormous and it is worth stating plainly: a drawn-in infield turns a meaningful share of ordinary ground balls into base hits, because the fielders have given up the two steps that make them fielders.
Why it is done, and when it is wrong. It is a scoreboard decision and nothing else. Tied or down one in the late innings, the run on third ends the game or ties it, and preventing it is worth conceding hits. Up three in the fifth, that same run is worth almost nothing and giving up extra base hits to prevent it is a straightforward error. The halfway version, corners in and middle infielders at double play depth, is the honest compromise: it keeps the double play available, which ends the inning and the threat, while still shortening the throw home from the lines.
What people get wrong about it. Watch which way the infield moves after the first pitch of the at bat. A defense that comes in and then goes back is reacting to the count, because a hitter ahead in the count is more likely to lift the ball and less likely to hit the ground ball the alignment was designed for.
At Little League and travel ball. Youth teams bring the infield in far too often and far too early, usually in the first inning of a game that will be decided by six runs. The play at the plate is also much harder than it looks at those ages, because it requires a catcher who can receive a throw and apply a tag, which is a advanced skill.
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