When it comes to skiing, it seems that up to three people can be talking about the same thing without using any of the same phrasing, concepts or perspectives. Also, there comes a point where the complexity and subtlety of our personal/internal understanding of what our own bodies are doing is not very applicable for linguistic distribution. So, I'm not exactly sure that I am experiencing a fundamental disagreement. Using a teaching system can help with that. What does the PSIA say about it?
If Rod is correct then that is what I would refer to as counter which is the absence of upper body rotation with the skis. So, they are not leading but rather staying behind ... or "left ahead", depending on perspective. Not ahead but staying behind which makes it look "leading" on the skis' way back. It is not a movement but rather a suppression of movement such as the difference between "separation" and "rotation".
While there are many vertical layers of rotational ability within the body, primary rotary separation is at the waist and, IMO, all other rotary abilities are either in support of that primary rotation and/or insignificant. I think that an introduction to a usage of that third level of separation, something I am familiar with as "shoulder carriage", is a technical can of worms to a very high percentage of skiers. I believe that a concept of using shoulder carriage rotary separation to lead lower torso rotary separation is initiation in the wrong direction.
I think the rotary separation dichotomy to strike here is that the "shoulder carriage" (neck, shoulder, arms) can rotate over torso rotation for slight adjustment or support of torso counter. There is no need to "power" torso rotation movement with arm swing inertia which is what Ligety is doing within his specialized realm.
This is the way I see it based on and consistent with my personal and foundation of understanding that is completely independent of any one teaching or coaching system.