Board Thread:Theories and Speculation/@comment-26138683-20150322132758/@comment-2120712-20150323122622

Yin is the spiritual energy and yang is he physical energy. Most of the time when these energies are combined they cancel out each other's special properties and become regular chakra. Some ninja have trained on how to use these energies individually, aka genjutsu users being users of yin release and akamichi using yang release to burn their calories into size enhancing chakra. And even fewer amount have learned to combine these energies while still keeping their unique properties, aka Yin-yang release. Most of time(99.9%) when these are merged they lose their special attributes which results in regular chakra, other factors like genetics may change the chakra or its malleability towards certain chakra transformations, aka nature transformation based kkg or uchiha being affiliated with fire release. It seems the ability to merge both into yin-yang release was in some way's Kaguya's elemental kkg that passed down to her son's but due to how Hagoromo changed the chakra structure of his sons + the blood line thinning, the ability to merge the two without them canceling each other was lost. Only by the sage granting some form his power himself to another or going through the arduous task of becoming the Ten Tails jinchuriki can one gain such a ability. In theory, yin-yang release is the hardest nature transformation to obtain.

So in short, focus training on spiritual energy = Yin Release focus training on physical energy = Yang Release Mastery of both + some form of the sage's(or just his generation's otusuki) power = Yin-Yang release.