The Yin Devouring Demonic Technique was rather rare even in Purgatory. It possessed two parts.
A Demon could acquire Yin Energy through its first part by raping a human woman. This wasn't a rare ability. Many demons could do this even without this technique.
There was a second part to this technique. And suppose a demon possessed the second part of this technique. In that case, he could absorb a Yin Energy even from a live individual or from a technique and utilize it to recover his injuries or bolster his Ardor. In other words, an attack filled with Yin Energy would be useless against those possessing a complete manual of the Yin Devouring Demonic Technique. He would instead be healed by that attack!
Azaroth had searched for this Yin Devouring Demonic Technique and even found both parts after he reached the Demonic Emperor Rank.
However, only then did he realize that this Yin Devouring Demonic Technique was more or less useless for him.