this is what I see.
1 - The left wrist radially deviates from a weak grip which shows up as dorsi flexion at the top of his swing. (The infamous cupped left wrist).
2 - The right wrist also radially deviates and shows up as dorsi flexion at the top of the swing.
3 - The right arm/elbow stays close to the body on the BS and allows the hands to move up into the top position all while rotating the body. This allows the left arm to be straight and go up and across the chest and shoulder line.
4 -
In the transition, the left wrist has ulnar deviation that releases the "cupped" wrist and puts it into a palmar flexion. This is where Hogan says he supinates or bows the left wrist on the way down. There maybe some left forearm pronation to lay off the club from the top too. This pushes the right hand/right elbow down and into the body. The so called "free ride down". Some like to call this the gravity drop but I don't think Hogan just let it fall. This would involve timing and waiting. He pushed down with the left wrist and pulled down with the right elbow. This got his right elbow on his right hip.
5 - The right wrist ulnar deviates slightly but keeps the dorsiflexion in the down swing. This keeps the club face from ever flipping at impact.
6 - From there he just freezes the triangle of the arms/wrist and just body pivot rotates the hell out of it.
There, I like my own description of what I think Hogan does better than how the 5L book describes it.
For those who wonder about my radial deviation that turns into dorsiflexion statement, try this experiment.
Put your left arm out in front of you like you are gripping a club with a weak grip (thumb on top). Now take your thumb straight back toward your forearm (I remember someone saying Hogan could touch his forearm with his thumb). You don't have to do it that far. What happens? Not only do you radially deviate, you also now have dorsiflexed (cupped the left wrist). Hmmm, the secret to the secret (cupped left wrist)?
Talk about finding nuggets of gold in a trash heap. This is a great thread if you can get over the signal-to-noise ratio.

Hogan had very late release and the cup in the left wrist is still there when the hands are in front of the right thigh. Check the gifs.
To the extent there is any move from cupped left wrist into palmar flexion, it happens in an instant, very late in the DS... not in transition.