The reference is about the poison that made Tamahome's ファン cry in the Tankoubon 5...In reality is not something to control the other, but to kill. But well, at first Nakago 発言しました he'd used the poison, but actually had thought to used another type of drug. The fact is that it was gonna get very complicated, so I left it like that...
About the elaboration of the poison, it's made of poisoned bugs...gosh, I don't know what I'm doing explaining this to あなた (How am I supposed to know how it was made of?!) Anyway...and about that incense that made Tamahome feel dizzy, it's actually an oil with a fragance extracted from the body of an animal, and it's very normal. It doesn't have any special effect, but...
1. Despite of calling it incense, it is something else.
2. Like Tamahome is poor, he'd never smelled a fragance like that and that's why he fainted.
3. I used it because I want it!
The reason is in one of this three options.
Is there actually a sex technique?
Yes, あなた shouldn't understimated the Chinese. The sex technique is based on the theory that men have the yan energy and women, the yin energy. So, through the yin energy of the women,men strengthen their yan and manage to become partly inmortal!! Well, they 発言しました that with this, men lead the other person's energy to obtain good health (I wonder if that's true, but...as an ultra secret technique it made me think that's really possible). Therefore, is not something strange at all. Nakago has Soi as his sex mate to strengthen his energy.
Taken from book 9 of F.Y.
