The Six Paths Yin Power is a tattoo that was created by Hagoromo Ōtsutsuki and part of a powerful sealing technique.
Background
Besides being comprised of the Yin Release nature transformation, the actual applications of the seal aren't clearly understood. What is known, is that it allowed Sasuke Uchiha to bisect Madara Uchiha clean through as the Ten-Tails' jinchūriki without much effort despite the fact that he was mostly susceptible only to senjutsu-powered attacks and strong physical force.
It has also been noted by Black Zetsu that there is a special resonance between the Six Paths Yin and Yang powers, making it easier for the wielders to sense each other.
