Ive never heard of 'separating the ribcage from the abdomen' either. I am curious how this could be used for power generation because I dont understand how it could be, but am open to different methods of power generation if it makes sense, can be taught and replicated, and works.
as to the subject at hand...
all good (ie. based in reality) 'internal power' methods are based in sound physical and biomechanical principles.
The way I might interpret it is that as an isolated movement, it is not going to do much. But that as a component of the, for example, ideal/abstract bodily states of 'open' and 'close', it is a critical part of actively carrying power through the area and also of generating power for the other parts to carry through as well. Your trunk is a big flexible ellipsoidal rod... thing, and the surest way to kill its potential is to treat is as a big stiff block. I think it may have been said elsewhere somewhere, that morning glory/asagao is in the body, not in the hand, per se.