Forearm muscles!!! See 6-B-2-0, Homer got it right. Also see 1-F, the Right Forearm is always driving!!!
DG
The Right Forearm is being driven by the Pivot in Swinging and by the right triceps in Hitting. The Right Forearm can make a slapping motion with its muscles using the right elbow as the pivot point, but doing this from the Top would only throw away the left wrist cock, which ZJ is clearly not doing.
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