It's hard to do, honestly.

The windup gives a pitcher a "cheap source" of velocity in the form of forward momentum.

But proper mechanical form should not differ between the stretch and windup.

All motion prior to the balance point is simply for rhythm.

From the balance point on, the stretch and the windup should be identical.

The trick is to harmonize the consistent balance point with a sub-1.3 second delivery.

Practice in front of a mirror until you've got it.

Work on it!