EXPLAINER: What exactly is a check-swing?

It's always been one of the most murky areas in baseball: What exactly constitutes a check-swing? Suffice to say, a lot of people were taking a swing at that one Thursday night after San Francisco's Wilmer Flores was ruled out on precisely that kind of call, ending the Giants' 2-1 loss to the Los Angeles Dodgers in the deciding Game 5 of their NL Division Series. Don't bother looking for it in the Official Baseball Rules: Amazingly, in the nearly 200 pages that govern Major League Baseball, there's no mention of how to decide.