What is a check swing?

Seems like the Cubbies let some great chances slip by, though I didn't see the game. Not sure I want to, knowing the result.
Caught most of the A's-Red Sox, though. What a ballgame! A's have to feel real good stealing one when Pedro was starting. Pedro was running on fumes his last inning, not even able to get his fastball up over 90mph to Durazo.
Something I learned this year is that there's no rule in the Major League Baseball rule book defining what a check swing is. The umpire just has to make his own judgment as to whether or not the batter swung. So when Durazo checked his swing on Pedro's changeup to load the bases and the third base ump said he didn't swing? How did he determine that? And when the TV cameras show the check swing in slow motion, how are we supposed to determine if it was a swing or not? How has this not been settled definitvely by Major League Baseball? Amazing. Personally, I'd propose that the bat has to cross the third base line (for LH batter) or the first base line (for a RH batter) to be considered a swing. You could use the front of home plate as a tighter boundary but then neither the third or first base ump would have a good perspective on it.
Great thing about the playoffs, especially a 5-game series: every game is critical. Inevitably, if the home team loses game one, everyone comments on how their backs are against the wall and how they'll have to win three of the next four. All those types of comments get old--I mean, we can all add. Still, the tension it adds to every inning renders just about every playoff game watchable.