THE PLATE UMPIRE SHOULD NEVER MAKE A CALL ON CHECKED SWINGS. The checked swing is still one of the murky areas of baseball. It deals with the situation where a batter stops his swing. The rule is that if there was a swing, the pitch was a strike and if there was no swing, it was a ball. However, there is no definition of a “swing”. As this wikipedia entry says, “The Major League Baseball rulebook does not contain an official definition for a checked swing, merely stating that a swing either does or does not occur….” The entry lists several factors which umpires take into account from time to time in deciding whether there was a swing.
Another problem is that the plate umpire can make the call on whether there was a swing. And he does make the call some of the time, even though his main job is to decide whether the pitch is a ball or a strike. When he does so, he is admitting that he was watching the bat rather than the ball. I remember when the plate umpire was the only umpire who was permitted to decide whether there was a swing. The rules were then changed to permit an appeal to be made to either the first base umpire or the third base umpire. Presumably, the change was made because there was understandable dissatisfaction with what happened when it was the plate umpire who decided whether there was a swing. However, the traditionalists won an exception—the plate umpire was still permitted, if he chooses, to make a ruling that there was a swing.
This is another area where umpiring will change because replay exposes inconsistencies.
(I glanced at Google and found this quote from Nico in a post at the Oakland Athletics blog on the SBNation site: “It is absurd, in my opinion, for the home plate umpire ever to judge a check swing. For him to do so is essentially an admission that he was focused partly on the bat at precisely the time he was supposed to be watching the pitch.”)