Plundering a woman's Yin

The Eternal Twin Demon Technique which Bai Long gave to Cao Ning and Zhenya was a kind of technique which he considered to be similar to what the members of the Yin Plundering Sect used.

But only after seeing Meirong plunder a woman's Yin in front of him did Bai Long realize how wrong his thoughts were. 

With Cao Ning and Zhenya, the way they cultivated was by letting their bare backs touch each other and use it as a contact point make Qi travel back and fro through each of their bodies.

However, the Yin Plundering Sect's technique stayed completely true to its name.

Once she stripped the woman naked and made her lie on the floor with her back pressed against it, Meirong also stripped herself without caring about the many people in the room seeing her naked. 

After she turned completely naked as well, Meirong lowered herself on top of the naked woman's body whose legs she had spread with her arms.