Granted the rule is terrible because semantically it's based on the batters intent, which is impossible to judge - in fact I would say that based on intent every checked swing should be a strike because at some level the batter meant to start swinging the bat which to me meets the definition of 'offering' at a pitch.
But practically speaking, while the rule doesn't provide an objective standard, the umpires have more or less created one, which is whether the bat crosses over home plate. (and I think this is reasonable because if the bat gets that far, it could have hit the ball.) I think many fans think it's more poorly defined than it is because they tend to look at the batters rotation, and the batter often stops his rotation but still cross the plate. On that basis the rule seems to be as clearly understood and enforced as well an anything the umps are responsible for - which granted - may not be saying all that much!