Thread:Ash9876/@comment-5778280-20160907021832/@comment-3030647-20160908125748

Lol well this is my own definition of Onmyoudou so don't take this as fact.

But basically, reishi is constituted of two parts: a fragment of a deceased soul, and the pure energy in the particle.

Tapping onto energy sources such as Ley lines, which go through a filtration process - and hence appear underground - means that one's body can be used as an anchor to flood this energy through. This is especially effective if one is a Quincy. By purging one's own spiritual energy temporarily with the pure energy source of the Ley lines, a Quincy's natural spiritual power (that destroys Hollows and other malevolent entities) is temporarily overrided and thus the Hollow is not killed, but reincarnated. (That's the gist of it, anyway.)

"Impure" is basically when energy in its purest state is tainted by the willpower of a soul. Although, this isn't actually a bad thing, because it's how most spiritual beings fight (Kido, Hakuda, Zanjutsu, Hoho all run on the philosophy of will imposition in one way or the other.)

The problem of impurity in the context of Onmyoudou is that it isn't an art that imposes will. If I momentarily take the description from Kaiba's page, it's basically:

By utilizing spiritual mediums such as ofuda or their own body, Onmyōji can act as a "rock in the rapids", ultimately redirecting energy flow in the same way a catalyst enhances a chemical reaction without getting consumed by it.

Because Onmyoudou works on energy flow, redirecting energy that does not abide by the typical flow is hazardous: energy that doesn't abide by the flow is energy that possesses a strong will concentrating it. (Of course other factors come into play but this is the gist of it.)