rojcewiczj wrote: My current understanding is that it has to do with having space within oneself, or to reside within ones own space. Meaning, that one has the space to change internally regardless of external restrictions.
That
might be a result of song, but not what song is. You've made this much more complicated than it is or needs to be.
It remains very mysterious to me.
It isn't at all mysterious. Bao's description of it is a good one.
Song, itself, is a very simple concept: relax, allow things to loosen and lenghten, eliminate excess/unnecessary muscular exertion. It is primarily physical and experiential. It is not academic. As Bao points out, one can't "think" one's way to what it is or its relevance.
If we say Song means to relax or release, How does relaxation effect application?
That is a central question.
A Traditional axiom of nature is that the strong beat the weak, the fast beat the strong and the young beat the old. Taijiquan is a method for reversing that axiom. In essence, your question is really, "What is Taijiquan's method?" Being/achieving "song" is central to that method. Books have been written on what is Taijiquan's method: it is the classical study of Taijiquan.
In my opinion, song/fang song is the most difficult skill/ability to achieve, without which the traditional skills of the rest of the art aren't accessible. By contrast, being "hard", "muscular" or "forceful" are abilities that most people innately have or learn. That is why so much of the art is focused on learning the "soft" skills: for most people that ability, and its application, is not innately present, and must be explicitly trained. If you start reading any of the classical texts on Taijiquan, you won't get very far before running into some skill that relies upon, or explicitly references "song".