Whenever you leak vital spirit, being stirred and interacting with beings, that is all fire. Whenever you gather back spirits consciousness and quiet it down to steep in the center, that is all water. When the senses run outward, that is fire; when the senses turn around inward, that is water.
The one yin [ inside the fire trigram ] concentrates on pursuing sense experience, while the one yang [ inside the water trigram ] concentrates on reversing and withdrawing the senses themselves.
Water and fire are yin and yang, yin and yang are body and mind, body and mind are spirit and energy. Once you withdraw to rest your vital spirit and are not influenced by objects, then this is true intercourse, as of course when you sit in profound silence.
Reference:The Secret of the Golden Flower: The Classic Chinese Book of Life new translation by Thomas Cleary XI p. 55