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It's a rock and roll motion, Doc. And smooth on the rock.
He rocks back onto that ack leg, gets into that straight line position, and then just unrolls to the plate: Leg, hips/shoulder, arm.
I'm glad you mentioned the glutes, too.
This is smooth-easy/powerful-low stress (as much as 100 MPH can be).
Sam Snead, if he threw a ball. Or Freddy Couples.
Those kind of motions last forever.