See for Yourself. Woman's Chi and Energy, Goddess of Sacred Sex.

So its best to wait until the water in the woman's womb is about to boil before beginning intercourse. The traditional Taoist texts mention nine things to look for in a woman as a sign she is truly ready for the man: A woman's energy goes through nine stages as her organs begin to warm and release their harmonious chi.

It was described by the Mysterious Lady, a sex consultant to the Yellow Emperor of China nearly two thousand years ago:
1. Her chi is in her lungs when she begins to breathe rapidly.
2. The energy has moved to her heart when she kisses the man. 3. She hugs the man: chi is in the spleen.
4. Her vagina grows moist: chi is in the kidneys and genitals.
5. She moves her pelvis and bites gently: chi has penetrated the bones.
6. Her legs grab tight around the man: chi is in the muscles.
7. She caresses the penis: chi is in the blood.
8. She kisses with deep passion: chi has arrived in her skin and flesh.
9. She surrenders herself and moans in ecstasy: chi has entered the liver and released her spirit. She is truly ready to receive the man and exchange her yin essence with his yang essence.

The purpose of waiting until the woman is fully ready will become clear when you practice the semen retention technique taught in the next chapter. The man and woman begin to "steam up" the sexual essence, the ching chi, but if the pot is not properly prepared and the temperature is wrong, it will be difficult to successfully transform the sex essence into spirit.

If the man lets his fire go out (by ejaculating his seminal fluids) before the woman is ready to begin the steaming process, then it will be impossible to refine the male and female essence together into a single nectar.

~John Moore ~