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The principle of Yin and Yang is that union leads to gathering, while disunion leads to separation. Union results in success, while disunion results in failure. This applies to both the Way of Heaven (Tian Dao) and human affairs, but is particularly paramount in the matter of procreation. The key factors regarding union and disunion are ten in number; if one can grasp them, the initiative lies with oneself.

First is "He-Pi" (Opening and Closing). This is the motive force for conception in women. When Qi is still, it closes; when Qi moves, it opens. Movement arises from the arrival of Qi, like a giant whale drinking from a river, or a large goblet being emptied drop by drop. At this moment, breathing is natural, and one is unaware of how the essence enters. Therefore, there has never been an opening that does not receive, nor a reception that does not result in pregnancy. However, this opportunity exists only for an instant. If one acts before the opening, it is too early; if one acts after the closing, it is too late. At this juncture, there is a subtle state of resonance that can be intuitively understood but not fully expressed in words. Only those who are attentive can perceive it; like rain falling from clouds, few fail to conceive.

Second is "Chi-Su" (Slowness and Speed). This is the timing of intercourse between man and woman. Slowness should meet slowness, speed should meet speed. However, the constitutions and temperaments of Yin and Yang vary. Those who are rigid react slowly, while those who are lax react quickly. If the slow partner dislikes the fast one, it is like hunger waiting for food, yet unable to swallow when it arrives. If the fast partner fears the slow one, it is like being drunk and wanting to vomit but unable to do so. When speed and slowness are mismatched, they cannot harmonize. To encounter speed with slowness, one should employ surprise tactics without revealing intentions beforehand. To encounter slowness with speed, one should wait quietly, stimulate first, and then engage, thereby reversing the situation and meeting the opportune moment.

Third is "Qiang-Ruo" (Strength and Weakness). This relates to the psychological dynamics of fear and submission during intercourse. If the male is strong and the female weak, she may flee as if avoiding stinging insects or weapons. If the male is weak and the female strong, he may collapse at the sound of wind or retreat at the sight of dust. When strength and weakness clash, it is rare for their paths and hearts to align. However, soothing the weak requires benevolence and righteousness, aiming to win her heart. Overcoming the strong is difficult; unless one gathers spirit and focus, how can one captivate her soul? Thus, the strong need not be overly feared, nor the weak overly worried; it depends entirely on the practitioner's approach.

Fourth is "Yuan-Jin" (Distance and Depth). This is the timing of physical union. Some, due to large stature, may intrude abruptly, causing discomfort; others, through stealthy glances, may dare to penetrate deeply. The desire to resist may be impossible, while the desire to accept may be unfulfilled. Such separation makes harmony difficult. However, restraining form relies on appearance, while reaching far relies on Qi; restraining form is momentary, but cultivating Qi takes time. Without long-term cultivation and planning, one may lack the vigorous energy needed to penetrate deeply and achieve union (symbolized by the dove occupying the magpie's nest).

Fifth is "Ying-Xu" (Fullness and Emptiness). This is the vital mechanism of men and women. The stomach has fullness and emptiness: full after eating, empty when hungry. The Kidney (Shen) has fullness and emptiness: full when storing essence, empty when releasing it. Prosperity and decline, success and failure, all depend on this. If one does not know how to utilize this balance, one may achieve accidental success but lose consistency.

Sixth is "Lao-Yi" (Labor and Rest). This is the Qi mechanism of men and women. Labor scatters Qi, leading to timidity; rest gathers Qi, leading to firmness. This can serve as military strategy for breaking enemy lines or as agricultural tools for planting. Acting appropriately leads to victory.

Seventh is "Huai-Bao" (State of Mind/Emotion). This is the emotional mechanism of men and women. When emotions align, they unite; when they conflict, they separate. Joy belongs to Yang, so those with abundant Yang tend to be joyful; depression and anger belong to Yin, so those with abundant Yin tend to be angry. Abundant Yang generates life-giving Qi, while abundant Yin generates killing/destructive Qi. These life-and-death Qi dynamics determine the potential for nurturing wise or foolish offspring. One must know which path to follow.

Eighth is "An-Chan" (Hidden Miscarriage/Subclinical Loss). This represents missed opportunities by the male. Do not assume that because you are strong, you have nothing to worry about regarding heirs. Do not think that because you are young, indulgence is harmless. Ignorance of excess leads to missing the prime window. Even the strong may lose vitality ("sour taste"), experiencing frequent losses without realizing it. From once to twice, to three times, the condition persists. Previous conditions involving spontaneous abortion should also be examined.

Ninth is "Tong-Zhi" (Childhood/Immaturity). This refers to the timing for women. Just as buds have not yet opened and vital Qi is not fully extended, or just as a girl has reached puberty but Tian Gui (menstrual essence) is not yet abundant, can immature seeds serve as good stock? Can underdeveloped silkworms spin cocoons? Forcing effort while aging is futile. This demonstrates a lack of understanding of timing.

Tenth is "Er-Huo" (Two Fires). This is the Yang mechanism of men and women. The Minister Fire (Jun Huo) resides in the Heart (Xin), which is the sovereign; the Ministerial Fire (Xiang Huo) resides in the Kidney (Shen), which is the root. These two fires interact, responding to every stimulus. Therefore, the Heart should be calm; if not, fire moves due to desire, stirring the Kidney from above. Starting with the Heart and then affecting the Kidney involves Yang burning Yin, which is forced and unnatural, causing Qi to sink. The Dantian loses its hold, losing the true nature of Yuan Yang (Original Yang). The Kidney should be sufficient; when sufficient, Yang rises from below, moving from Kidney to Heart. Starting with the Kidney and then affecting the Heart involves Water nourishing Fire, which is natural. In nature, Qi leads, and all meridians arrive together. This is the true mechanism of creation. However, frail men often rely on force, leading to deficiency consumptive diseases, lacking heirs. Robust men follow nature, ensuring prosperity. Those who understand timing must grasp the true mechanism of Yang.