Appeal Plays
The defense has to ask for the out: a missed base or leaving early on a fly.
How it actually works. A runner who misses a base or leaves too early on a tag-up is not automatically out. The defense has to put the ball in play and touch the base or the runner while telling the umpire what it is appealing, before the next pitch and before the pitcher leaves the mound area. If it does not, the run counts and the base is treated as touched.
Why it is done, and when it is wrong. This is one of the few places in the rulebook where the umpire is required to watch something and then say nothing about it until asked, and it exists because the alternative is umpires officiating a footrace on baserunning technicalities. The practical consequence is that a coaching staff has to be watching every runner touch every base, all game, on the chance that one of them does not, which is why you occasionally see a first base coach shouting about something that happened two plays ago.
What people get wrong about it. The window closes fast and it closes in ways that surprise people. A pitch, a balk, or the pitcher stepping off and disengaging from the mound area can all end the chance, and an appeal made incorrectly can be used up entirely.
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