The first time when I saw how a neidan student relate yin yang to his practice and the Book of Changes but fumbled, I had wanted to comment but I said nothing. Later the same student perhaps after checking with his teacher from a lineage in China tried to tie up the relationship between yin yang, earth and heaven but still could not convinced the audience (some discerning or skeptical Tao Bums) why a neidan practitioner cultivates towards yang instead of yin.
A few months after that, I see Yi aficionados discussing yin yang in relation to interpretations of prognostications in an I Ching Forum.
Nothing wrong with that too, except that it could complicate our studies and our explanations to self and others, if we try to be sophisticated instead of keeping it simple and easy.
The holy sages who made the Book of Changes also made it simple and easy. That is why they determined the Tao of Heaven to be the dark and the light. And they determined the Tao of Earth to be yielding and firm.
We can easily substitute yin yang for the dark and the light, yielding and firm, earth and heaven respectively. But is that what the ancients taught? And can we or others see yin yang or understand the concept with ease?
If the particular neidan student or his teacher had given it a deeper thought, it could have made understanding for both the transmitter and the receiver(s) a bit easier.
Is it not easier for the audience to understand that a neidan practitioner cultivates Tao to become a light being (an immortal, a divinity), instead of a yang being (even though the term light is often substituted by the term yang by Daoist adepts)?
If we learn to keep our Yi and/or neidan studies and practices simple and easy, we may one day emulate earth and heaven. But how would I know?