 In Shangqing Neidan "Internal Alchemy", Baldrian-Hussein says,
the "po" plays a particularly somber role as it represents the passions that dominate the "hun".This causes the vital force to decay, especially during sexual activity, and eventually leads to death.The inner alchemical practice seeks to concentrate the vital forces within the body by reversing the respective roles of "hun" and "po", so that the "hun" (Yang) controls the "po" (Yin).