I do not have the feel that you do. I do not feel my trial hand go 'below'. In fact I'm quite sure if I felt this I'd be doing it incorrectly as this is closer to the feeling I get for breakdown of my lead wrist.
I do relate to what Petearm was saying but if I am it bears no relation to feeling In the first case I feel the trail hand going below the lead hand through ?impact' but rather the opposite if anything.
Bertholy observes that the lower body spins (as well as producing leverage) but does NOT say that this is something you try to do. I repeat - There is NO - EXPLICIT - ACTIVE rotation of the lower body. The only EXPLICIT - Active action of the lower body in the downswing is the movement of the lead knee. There are a number of details on positions you should be in at impact for example and you 'rehearse' these as poart of the excercises and similarly with these there is NO direction re hip rotation except that rotation should be limited in the backswing.
I do not think this to be an accident or an oversight.
Peter