[DownTownTonyBrown]

Following Rule 10-1-4 Federation rules make the following comment:

NOTE: The umpire-in-shief- sometimes asks for aid from the base umpire when there is a question as to whether a batter's "half-swing" is such to be called a strike. As an aid in deciding, the umpire may note whether the swing carried the barrel of the bast past the body of the batter, [b] but final decision is based on whetehr the batter actually struck at the ball.[b] emphasis mine

As said by others, although this note give a possible method for helping make the decision based upon furthest extent of the bat, the final decision is to be based upon the umpire's interpretation of the batter's intent.

In general, the batter is obviously indecisive as to whether he should be swinging or not, so I help confirm his indecision. "Yes, you went."

I can't think of a time when I have been questioned for saying that the batter did go. But there have been numerous "situations" precipitated by saying no he didn't go. The defense always wants the strike called. The offense recognizes the indecision of the batter and generally tolerates the strike call.

As stated by those before me, the situation can also determine the proper call.