Cutoff Man
The fielder who lines up between the ball and the base on a long throw.
How it actually works. On any throw from the outfield to a base, one infielder goes out and stands between the fielder and the base the throw is aimed at. He is not a relay, he is a decision point: the throw is already going where it needs to go, and he can take it and throw somewhere else, or let it pass.
Why it is done, and when it is wrong. Watch where the cutoff man sets up and you know what the defense is worried about. Lined up with third base, they are trying to stop a runner taking an extra base; lined up with home, they have conceded that base and are protecting the run. A defense that gets this wrong does not lose the play by inches, it loses it by ninety feet.
The numbers behind it. Which infielder goes out depends only on which side of the field the ball is on. Ball to left, the shortstop goes and the second baseman covers; ball to right, they swap. On a throw to the plate the first baseman is the cutoff on anything from center or right, and the third baseman on anything from left.
What people get wrong about it. The commonest failure in youth baseball is not a bad throw, it is a cutoff man standing where he thinks he should be rather than on the line. If the outfielder has to change his aim to hit him, he is in the wrong place.
At Little League and travel ball. This is the first coordinated thing a young team learns, and it is why a well-drilled twelve-year-old team beats a more talented one. The talent is in the outfield arm; the wins are in the man standing in front of it.
Batter's Eye: look up anything in baseball