It seems every ump working the third- and first-base lines has a different definition of what constitutes a swing based on divergent understandings of the rule book (which, as noted above, has been almost deliberately ambiguous on the matter), although generally, these divergent definitions tend to favor the pitchers, who tend to get strikes on motions that don't resemble an offer.
It will be nice to see this standardized, but the system will clearly need some calibration and review of its own to work out. Umps seemingly try to read a feel for the batter's intent as much as a judgment of the batter's actions — which is a losing game, as the batter's intent is, by definition, conflicted.
It will also create interruptions in game flow and cause more degeneration into the videogamification of baseball.
It will also create a challenge for the batter, as starting and then stopping a swing — at a ridiculously fast pitch thrown by a ridiculously close pitcher — is a physically and mentally challenging act that you have to take a few beats to recover from, and having to make a high-stakes decision to pat your helmet while you are in the midst of that recovery seems like it could be tricky.
I assume you get to challenge a swing ruling from the home plate umpire as well as the base umps, correct?