Yin is creation energy. Yin comprises one half of an energetic circuit that drives creation in this universe. Yin energy is the formative half of the cosmic creation equation. Yin receives, and shapes, the powerful energetic thump of yang energy.
The human process of sexual procreation, from the genetic sex act itself, to conception, to the formation and birth of a new creation, is a perfect analog for the operation of the energies of creation. Explosive and powerful yang energies are received by an equally powerful yin-based receptacle, and creation occurs.
It should be noted that despite the fact that the female form functions as the biological manifestation of yin based functionality in the realm of pro-creation, it is absolutely incorrect to associate the female form exclusively with yin, or the male form exclusively with yang energies. That is, women are not just yin and men are not just yang. For reasons explained elsewhere, both the male and female physical unit require yin and yang circuitry, yin and yang energies in some adequate balance, to function properly.