Tagging Up
On a caught fly ball, a runner must touch his base before running.
How it actually works. The runner returns to the bag and stands with a foot on it, watching the fielder rather than the ball. He leaves on the touch, not on the ball entering the glove and not on the fielder's throw. From third base on a medium fly to the outfield that is usually a run. From second on a deep fly it is often a base. From first it is rare and it is almost always the fielder's arm being tested rather than the runner's legs.
Why it is done, and when it is wrong. The judgment is not about the depth of the ball, it is about the fielder's position when he catches it and how many steps he needs to throw. A shallow fly to a right fielder with a weak arm who is drifting back is a better tag than a deeper ball to a center fielder coming in on it hard. Coaches teach depth because it is easy to teach; the players who take the extra base read the fielder's feet.
What people get wrong about it. The most common baserunning mistake in the sport is being halfway between bases on a fly ball with fewer than two outs, which loses the base if it is caught and gains nothing if it is not. With two outs the runner goes on contact and none of this applies, which is why the number of outs has to be said out loud before every pitch.
At Little League and travel ball. Almost no youth runner tags up correctly without being drilled on it, and the reason is that they watch the ball. The single instruction that fixes it is to watch the fielder's glove and listen to the base coach.
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