Walk-off
The home team winning it in its last at bat, at which point the game simply stops.
What it actually is. A walk-off is any play that puts the home team ahead in the bottom of the ninth or later. The moment the winning run touches the plate the game is over, so the play is not completed in any normal sense: everyone walks off. It does not have to be a home run. A walk-off can be a single, a wild pitch, a walk with the bases loaded or an error.
What somebody who knows the game gets out of it. The walk-off exists because of the rule that the home team does not bat when it does not need to, and it is the clearest expression of home field advantage in the sport. It also creates the highest leverage situations in baseball, which is what win probability was invented to measure. A one run swing in the bottom of the ninth of a tied game moves the odds of winning by more than a grand slam in the first, and that fact is what the whole leverage index is built on.
The numbers worth knowing. Roughly a quarter of home team wins are walk-offs in some form. A walk-off home run in a tied game moves win probability by about 40 points on its own, which puts it among the largest single events a season produces.
What people get wrong about it. On a walk-off home run the hitter still has to touch every base, and his teammates are not allowed to interfere with that before he does. Runners ahead of him have to score in order too. Games have been reversed on this, which is why the mob at home plate waits.
At Little League and travel ball. Youth games end on time limits far more often than on walk-offs, and most tournament rules say an inning that starts before the limit is completed. That produces the youth version of the same drama: a home team trailing by one with the clock about to expire is playing its last at bat and everybody in the complex knows it.
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