So before throwing the hook in this manner, your weight in on your lead foot? Then as you throw the hook the rear foot slides outside a half-inch. And I assume this slide just facilitates the weight transfer? I will have to try this.
Try it, to start, after a right hand. Throw the right hand, then slide the right foot directly to the right an inch, inch and a half, then throw the hook. It works real well when throwing the hook to the body; when trying to pivot when hooking to the body, the weight tends to stay on the front foot. Slide the right foot over and you'll get the weight turned. Al Hostak, MW champ in the late 30s, used to use this method when throwing a lead hook, very successfully because it doesn't telegraph the punch. When you pivot on the front foot there is a tendency to wind up the shoulders that tips off the punch.
And I'd bet that's why he had so many back problems so early in his career. Just too much pressure on the spine. Must have been a calculated gamble by his team to plant so that he could deliver his hooks more efficiently.