Re: “I think it’s probably an interlink between the preceding prescriptive lines and the following introduction to the more primary concept of JIN.”
I think that’s a compelling idea. I definitely agree that the line about the many strands being made one thread is a transition between the preceding few lines and the following passage about the progressive stages of learning. I tend to think it is more of a summing-up statement referring to the idea of responding and following than an introduction to the next section, but you may be right. What makes it difficult to know with certainty is the lack of any sort of pronouns that may point one way or the other, and the ambiguity about whether “li” refers to a principle or a set of principles. Again, I think the ideas of responding and following in the preceding lines are supported by the “give up yourself and follow the other” line further down. Some commentaries mention that here, suggesting a link to that phrase as “the foundation,” hence the pervasive principle.
Still, the possibility that the “one thread” introduces the development of jin is interesting. In his Answers to Questions, Chen Weiming is asked what the difference is between zhuo (techniques) and jin. He replies, “The zhuo are the methods of transformation (bianhua zhi fa). Jin simply operates through the zhuo. There are myriad [variations of] zhuo, but there is only one jin. No matter what the zhuo, there is but the one jin; it is just that the intention varies at the time of application. Therefore the jin also complies with it and changes accordingly.” See: http://martialart.giss.ncpes.edu.tw/taichi95/books/B1929AW0/B1929AW00048072.jpg
A stronger argument might be made, though, that the opening and closing “threading” statements in the “Taijiquan Jing” have to do with jin and integrative movement—that the body “must be threaded together (guan chuan),” and “the entire body is threaded together joint-by-joint (jie jie guan chuan). The "one thread" line in the text we're discussing here, however, is talking about the underlying principle (li) of the art, which is a topic separate and distinct from jin, in my view.
Thanks to the the great site Gu Rouchen discovered, I've added a link to the page from Chen Weiming's _Answers to Questions_ from which I translated the above quote.
*Chen also explains "jie chin" (intercepting jin) on the same page.
[This message has been edited by Louis Swaim (edited 09-05-2008).]