Brocade has been found in tombs sealed airtight with clay and water dating back to the Fifth Century BCE (or there abouts) in what was then the Chu kingdom. Brocade is a type of woven fabric which can display different images depending on the angle of the light. See this Archeology PFD.
Probably the most widely practiced sinew lengthening routine is called 8 Silken Brocade. Following the metaphor, it has eight movements which reveal eight different images of what the human body can do.
The metaphor of the golden thread is absolutely key to understanding how to practice taijiquan. Each day when we practice we begin with some idea about what we are going to do. That idea may be very complex, or it may be as simple as "I'm going to make circles with my arm and try to make it look like it does when my teacher does it." But during practice, you happen upon some feeling that is new.
Perhaps on this particular day it is a feeling of connection, or a feeling of softness, it could be anything. The first time you feel it, it will feel subtle, even delicate, like a very fine golden thread. If you don't practice the next day, you will certainly have forgotten what the feeling was and how you got to it. Even if you do practice the next day, the feeling may elude you. Or you may refresh this new feeling for a few days, but then you have a wild night and the next morning you forget to look for that golden thread and by the next day it is gone. By the next week it is forgotten.
If you do manage to hang on to one of those new feelings (those subtle golden threads) for a few weeks of practice it will become less subtle and easier to reproduce.
To truly practice Taijiquan or any internal art is to look everyday for the golden thread you felt the day before and to slowly weave it day after day into the brocade that is your whole practice. Eventually these fine threads weave together into pictures that become so a part of you that they shimmer in and out of focus continuously like images on a flexible piece of brocade in changing light.