Oh the yin and yang of it all!
The concepts of yin and yang are not mystical, but instead are very straightforward and refer to two things which are opposite but also have a relationship to one another. That's it; no purple smoke and sparkles! For example, the two sides of a coin are different, but the common relationship is the coin. If we can see one side of the coin, it is logical to recognise there must be another side, and neither side can exist on its own. When one side faces up, the other side faces down; and both exist simultaneously.
When looking to differentiate substantial from insubstantial in the form, or in partner training, then the same concept applies. A simplistic example could be: If I place my palm on the top of my partner's wrist, and they then lift their arm, the uppermost part of their arm could be called 'substantial' (from my perspective) as it is moving towards my contact on their arm. Similarly, if I were to place my palm on the underside of my partner's wrist and they then lift their arm, the underside of their arm is moving away from my contact and therefore would be referred to as 'insubstantial' (from my perspective). So perspective is also important here, because in the first example a sensible (Taiji) response is for my hand to become insubstantial and yield to their upward movement without letting it build up force. Consequently, in the second example, a possible Taiji response would be for me to stick and follow their upward movement; i.e. my part would be substantial.
There are other ways in which substantial and insubstantial can manifest, but the fundamental relationship is the same.
The opposite of the 'heads' side of a coin is not nothing and so it is also important to recognise that the opposite of substantial (i.e. insubstantial) is not nothing. They are like two ends of a see-saw; when one changes, the other has a complementary change.
Paul Fretter, December 2021