"Well, the fact that it’s an analogy necessarily makes it conceptual, doesn’t it?"
Hmm, ok,but that may be water under the bridge.
"The movement resembles the opening of a fan, but of course mechanically the arms do not attach from a hinge in the spine. It is more a case of using the imagery of an opening fan to facilitate a feeling of connection between the two arms “through the back.”"
Sure, but I am just trying to suggest that the interpretation of the analogy depends on how it is conceptualized. For ex., your interpretation combines the idea of a "fan" with the idea of "through the back." The interpretation seems to be that mechanical action of a fan is correlated to the mechanical action "of" the back, not "through" it. By the way, I have no problem whatever with that description.
I posted the link to Jarek's site with Chen Fake doing "Shan Tong Bei." He translates it as "Flash Through Back" (iirc). Perhaps someone who studies Chen style might be able to suggest what the analogy is. Does something "flash" through the back --which could even imply a different image for a "fan" going "through the back." But, that might be over-literal.
"Also, I don’t think the fact that the movement is asymmetrical makes the fan imagery any less applicable."
I agree. I guess I am arguing that "fan" has more than one association.
"What I mean when I say that the Yang form is not “mere derivation,” is that while aspects of the form, and some of the form names, may certainly have derived from the Chen tradition, it is evident that something other than derivation occurred, given the obvious differences in the appearance and logic of the forms."
Oops, I certainly do agree that the Yangs were innovative. But, I was thinking about the actual names. I really don't know how closely the various styles follow the idea of "Shan Tong Bei". I was just saying that the name, itself, probably (since I could never say for sure) has the same origin. So, I consider their derivations to be the same. You are far more knowledgeable on Chinese language than I. However, in this case, I think it'd be unlikely that the Yangs deliberately re-invented old names or accidentally came up with names that were like the older ones.
Again, I do agree that the Yang interpretation of the names as manifested in the form is their own innovation.