It's a common Cuban style to start your hook with the thumb at the top and turn it inward and down just as it lands. They believe it adds more torque and momentum. The better of my two coaches never bought it, but he acknowledged how successful the Cubans are in international competition, "So they must be doing something right." I was taught to imagine you're holding a pitcher of beer in your hand. You don't want to spill it as your smashing it into your opponent's head.
In other words, you're not rotating your hand at all as you're bringing it around. I'm obviously not suggesting it's the only way -- just how I learned it and I pretty much stuck to it. That same coach was also big on longer hooks -- using them to reach targets further away than would normally be expected, like the kidneys.