My thoughts -
The start of the pitch begins with any movement that is normally associated with a pitching motion. In my opinion, the pitch starts when he move the hands from the chin to the chest and pauses.
Now I do not see a pause in-and of itself as a balk. However, once the hands move down from the chin, the pitcher is committed to throw to the plate.
If the pitcher does this every time or not is not a question that needs to be answered. Illegal is illegal, repeating an illegal motion on every pitch does not make it legal.
I believe that there is something that says the pitcher - once started - must make a continuous motion, but I am not sure that applies to the windup, the stretch or both. Gotta look that up tonight.
The balk is there to protect the runner from deceitful motions of the pitcher. If the pitcher is committed to the plate once the hands lower from the chin, then the pause, does not impede the runner in any way.
Also, I may be assuming something here, but if the pitcher is in the wind up - then there are probably no runners on. Not a situation to balk.
Would be interested to hear others on this.