It's a twist of the right forearm such as you would do if you were unloosening a screw with a screwdriver. Except the right wrist is bent when doing it in the Hit.
[Bold by Yoda.]
In the first edition of TGM, 10-23-E (Automatic Snap Release) read:
"The corkscrew effect of the straighening Right Arm, if unhampered, will whip the Clubface into Impact position and the Hands into Impact location as calculated in selecting the correct Aiming Point and Triggering Type (or combination) to accommodate an Automatic action."
Thanks for that 1st ed excerpt. Why has it disappeared from the 6th edition?
Homer Kelley's explanation of the Corkscrew Effect (of the straightening Right Arm) whipping the Hands and Clubhead into their Impact Alignments (Automatic Snap Release / 10-24-E) was maintained through the first three editions of The Golfing Machine.
Beginning with the 4th edition and maintained through the 6th (and last), he eliminated this all-inclusive term and referred instead to Paddlewheel Action (of the Hitter's Uncocking Right Elbow) and Throw-Out Action (of the Swinger's Uncocking Left Wrist). This change allowed differentiation (from Release to Impact) between the Hitter's Single Wrist Action(and its gradual Angled Hinge Rolling per 10-18-C-2) versus the Swinger's Standard Wrist Action (with its Swivel Action per 10-18-A).
In the as yet unpublished 7th edition, 10-24-E was substantially revised. However, the second sentence (referring to the Paddlewheel and Throw-Out Actions) was left unchanged.