Yin-Yang (陰陽 In'yō?) is chi, or energy, that flows through everything that is living and non-living.
Everything that exists possesses one chi of either Yin or Yang and are portrayed physically through gender: Yin being depicted through females and Yang being depicted through males.
The Yin and Yang cycle is implemented in Tao and increases it the greater the cycle becomes. The final training method of Tao, known as Bochu Jutsu, involves two individuals with different chi's to join in sexual intimacy to combine their Yin and Yang. Once done they gain a mastery of their Tao.
Lord Tensen's Yin and Yang cycle flows unbroken and as a side-effect their gender constantly switches to heighten their Tao.