Zhao Changhe had not known what this meant before, but having just learned the detailed exposition of the gold foil, he truly understood.
Dual cultivation is not narrowly about doing that sort of thing. Theoretically, even if you were to transmit a stream of True Qi to someone with your hand and they return it to you, it could still be considered a form of dual cultivation, and that's the broadest concept.
But if we talk about the narrow sense of Yin Yang Harmony, it's not done with hands. There should be Tai Chi intertwining as harmony—it must involve some intertwining, some opening, to allow for mutual transfer.
Of course, the mouth is the most typical opening on the human body.
And there's another…
It's best if both top and bottom connect, as that constitutes Tai Chi.
This is indeed the most orthodox theory, not just superficial.
But how did Yue Hongling know? Where did she get her knowledge of dual cultivation theory?