Back on Monday, Cubs manager Joe Maddon was calling for replay review on, of all things, so-called "checked" swing situations. Even the next day on the radio, he went further. Via 670 The Score: 

"There needs to be research done," Maddon said on the Laurence Holmes Show on Tuesday afternoon. "When you talk about technology, they want to reinstitute the strike zone. I don't want that. I think there's going to be a lot of unintended consequences there (with an electronic strike zone) that aren't going to be good. I don't think it's going to benefit the hitters as much as the hitters think. With the check swing, I mean technology-wise, there's got to be a method from an angle to determine how to laser when a bat exceeds a certain spot. 

"I'm thinking even implanting a chip in the head of every bat somehow that would permit this to be enacted."

This was on the heels of the Cubs having lost in extra innings Monday. In the top of the ninth with the Cubs in the lead by one and a runner on second, Andrew McCutchen was spared by the umpiring crew with two strikes on this non-swing: 

That would've ended the game, quite simply, had the first base umpire made a fist instead of holding out his arms in a "safe" signal. Instead, the Cubs later lost. A game of inches, as they say. 

This brings us into something that's long bothered me: 

There is no rule about what constitutes a swing

Seriously, that call above and every call that is appealed to a base umpire as to whether or not a player swings is totally just a discretionary thing because there is no rule. It's a bit astounding, no? Maddon's appeal to making technology available can't possibly come before MLB actually puts a rule in place. Think about it. What could they possibly review? There's no rule! There can't be clear evidence to overturn a call based upon something that doesn't exist. Scour the rulebook yourself if you want. It's 188 pages with zero mention of how to call a swing. The only mention about the situation comes here, in Rule 8.02c's comment: 

Appeals on a half swing may be made only on the call of ball and when asked to appeal, the home plate umpire must refer to a base umpire for his judgment on the half swing. Should the base umpire call the pitch a strike, the strike call shall prevail. Appeals on a half swing must be made before the next pitch, or any play or attempted play. If the half swing occurs during a play which ends a half-inning, the appeal must be made before all infielders of the defensive team leave fair territory.  

Again, this is astounding. It's recognizing there's a such thing as a "half swing," yet never defines it or gives any guidelines as to how an umpire is supposed to call it. 

Now, for decades and decades, fans, players, media, umpires alike have come to accept a general idea of what constitutes a swing ("did he go?"), but it's not defined in the rulebook. We can talk about breaking wrists or the head of the bat clearing home plate or the barrel passing by the front of the body, but all of these ideas came about through generally accepted word of mouth. None of them have ever been officially defined by Major League Baseball. 

Why? 

I've never been given a good answer. Not from anyone in a front office, anyone with Major League Baseball, any player, any scout, no one. 

This is far from the most pressing thing in baseball. It's not even a huge deal. This isn't me with sour grapes from my favorite team losing a game (they still should have won after the non-swing call). 

This is simply to highlight that the league should come up with a definition of how to call a swing. Even if it's something that is relatively vague (for example: "a player going more than halfway with the bat is determined to be a swing. Half or less than half is determined to not be a swing"). They define everything else, but something that happens multiple times a game should be defined.