Whenever the energy of the vital spirit leaks out, or the vital spirit moves and mixes with things, that is all Li (Fire) ☲. Whenever you gather back the spirit’s consciousness, quiet it down and hold it in the center, that is all Kan (Water) ☵. When the seven apertures (eyes, ears, nostrils and mouth) run outward, that is Li (Fire) ☲; when the seven apertures turn around inward, that is Kan (Water) ☵.
The yin （inside the Fire trigram ☲） follows the stimulation of the senses, while the yang (inside the water trigram ☵) reverses this and withdraws from the senses themselves.
Water and Fire are yin and yang, yin and yang are essence and life, essence and life are body and mind, body and mind are spirit and energy. Once you withdraw to rest your vital spirit and are not affected by internal and external stimuli, then this is true intercourse, as when you sit in profound silence.