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."