I would NOT worry about what the left forearm does. It moves as the Hands move. Standard Left Wrist Action turns on plane to the right on the Take-Away and rolls on plane back to the left on the Down Stroke.
The Hands, therefore the left forearm, does NOT move with the Hitter's Single Left Wrist Action, but the body rotation turns the Hands in relationship to the ball. How could it not? The Hands do NOT turn independently
The Hands, therefore the left forearm, does move with the Swinger's Standard Left Wrist Action. It moves independently of the body rotation. It may turn and cock early, mid or late in the Take-Away.
Forget about the forearm- Educate the Hands. They control everything.
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