Quarterbacking
The catcher runs the defense: signs, positioning, calls on every play.
How it actually works. On a ball hit into a gap or into a corner, the man fielding it has his back to the infield and cannot see the runners. Somebody with the play in front of him, usually the nearest outfielder or the trail infielder, calls the base out loud before the ball is picked up.
Why it is done, and when it is wrong. Almost every extra base given away at youth level is a communication failure rather than a physical one. The fielder had the arm and did not know where to throw, so he held it, or threw it to the wrong base. Naming the base out loud is free and it is the single most coachable thing on this list.
Batter's Eye: look up anything in baseball