The rules require the pitcher to step on to the pitchers plate with hands separated and maintain that separation for a discernible amount of time. The rules require the pitcher to pause after they have stepped on to the plate to take OR simulate taking a signal from the catcher. The pitcher can then bring their hands together.
The pitch starts when the hands separate. This is considered the "signal" to the batter that the pitch is imminent.
In my experience, most pitchers will wait, either off the pitchers plate or once they step on, for the batter to be ready.
If you feel that the pitcher is close to beginning the pitch when the batter is not set and ready, simply hold up your hand to tell the pitcher not to pitch.
There is already plenty of rules support that can be used in this situation.
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