This thread is a giant circlejerk, no one is giving the right answer because no one knows what it is. The wiki has a bullshit definition, it uses redundant terms and doesn't give you a real definition.
But to give you an idea of what it is, basically if you imagine stuff, it will happen. For example with shikamaru's shadow tech, under normal circumstances, a shadow couldn't do anything but if he uses yin chakra, he can make his shadow do stuff because he imagines so.
We don't know exactly how yin affected Sasuke's chidori, but we can assume he gave it more penatrative/destructive power, since Madara has the Juubi, a regular chidori wouldn't do anything. This might be related to his teleportation tech, he imagines him self over there and he appears there, he imagines his sword impaled in Madara, and it happens. Yin creates physical form.
Naruto's power is basically life-giving, so he can recreate body parts. An arm is complex, the nerve endings, and muscles and everything have to link to the brain perfectly. Normally if you just recreate a body part without yang release, an eye for example, it makes no difference because you can't see with the eye; it's just a bag of water. But Naruto used his mastery of yang release to "give it life" so it retains its function, that is to see. That's my understanding. Hashirama for example, was able to use wood release which was life, thus it required yang release mastery. Medical Ninjutsu requires yang release. Yang creates life.
in Naruto, there are more examples of Yin release than there are of Yang release.
