The phrase sounds mystical. It isn't. It's a precise description of mechanics.
If your opponent attacks with force (Yang), you become soft (Yin) — not to absorb the blow, but to redirect it. Redirect a vector rather than opposing it, and you need very little energy to neutralize a great deal.
This Yin-Yang principle runs through every movement: at any moment, one leg is "full" (weight-bearing, active) and the other "empty" (light, ready to move). Inhaling is Yin, exhaling is Yang. A closing movement is Yin, an opening movement is Yang.
This constant alternation isn't optional — it's the structural principle of every form.
The Tai Chi symbol isn't just a logo. It's a description of how the body is trained to move.