In Asian philosophy Yin is one half of a whole, this being Yin-Yang. While the two represent a single whole, they are also complimentary and contradictory forces in the universe and spirituality. It is a duality that simultaneously opposes itself, but comes together to form a perfect balance that is constantly in motion. Yin, specifically, is characterized as dark, fluid, and docile. It is also seen as passive and feminine, as well as represented as the moon.
Yin and Yang is present in many Asian philosophies; these are most notably Confucianism and Daoism. Because Reborn studies Daoism, and it was used as the basis for Yin on NB, I'll focus more on that rather than Confucian Yin (even though they are very similar). Daoism focuses on the balance aspect of Yin and Yang. It notes that the two forces are always flowing and never in a static state. In the Daoist philosophy, Wuji is the primordial universe; it is essentially 'nothingness' or the void. From nothingness came everything, or Taiji. But because there is no fundamental difference between nothing and everything, polarity is introduced in Taiji, which forms Yin and Yang. From Yin and Yang come the 8 Trigrams, and through that come Creation of All Things (10,000 Things).
Yin and Yang cannot be separated and exist independent of one another. This is represented by the Taichi symbol, which has Yin present in Yang, and Yang present in Yin. So while they are contradictory forces, they also sustain one another. They are opposite, but balanced together.
Now, in the context of Naruto Yin is the energy that governs spiritual energy, the imagination, and darkness. In simple terms, it is creation from nothing. Birth from imagination. Form created from lack of form. Yin is the application of imagination; because of this, it is capable, on its most basic level, of creating characteristics that would otherwise be impossible in techniques. Yin can grant fire the property of being cold, it can cause lightning to become solid, or make earth intangible. It has powerful chakra draining qualities, notably in the Phantom Dragons, Hands of Sloth, and Hungry Ghost.
Beyond this, because Yin governs the spirit and spiritual energy, it is intrinsically connected to the mental state. It affects the minds of others, or even the user's, as well as the balance of spiritual and physical energy in the body. Yin's nature allows one to take Genjutsu techniques to a level beyond what would normally be possible. Illusory techniques can be empowered to bend the will of the user's victim, weaken their spiritual energy and wipe out the balance between spiritual and physical. One of the most important aspects of this is that, because the balance between spiritual and physical energy in a body is thrown off, Yin can render someone unable to mold chakra.
A lot of fields stem from Yin. The obvious one is Genjutsu (as previously discussed). Yin is inherently the basis for Genjutsu; it affects the spiritual energy of a target and can bend their will and throw off the balance of energies. Yin also forms the basis for Nara and Yamanaka techniques, both being spiritual in nature. But its important to always remember that Yin is vastly larger than these. These are products of Yin. But Yin at its core, and the reason for why I picked it as my starting energy, is that it is creation from imagination. Form from nothingness.
