The Six Paths Yin Power is a power wielded by Hamura and Hagoromo Ōtsutsuki, and later given to Sasuke Uchiha. It is represented by a dark-coloured, crescent moon-like marking on the user's left palm.
Besides being comprised of the Yin Release nature transformation, the actual applications of this power aren't clearly understood. What is known, is that it enabled Sasuke to infuse his Chidori Sharp Spear with the Six Paths power, allowing him to bisect Madara Uchiha as the Ten-Tails' jinchūriki. Which one can presume that it auguments the user's abilities.
As noted by Black Zetsu, there is a special resonance between the Six Paths Yin and Yang powers, allowing the wielders to sense each other's presence. When the Yin and Yang powers are used together, they can transform their target into the core of Chibaku Tensei. When Sasuke did so in conjunction with Naruto Uzumaki's Six Paths Yang Power, the mark on his left hand vanished, returning to Hagoromo's own left hand.