I had someone ask me a question in regards to self-mutilation as it pertains to shamanic practices in Aesca, at least in regards to Ten-Ghost. The question was, if I recall it correctly, “How come Ten-Ghost can sew a finger back on, but my character can't make a prosthetic eye for a shaman I know?” The first part of the answer could be summed up as “Your character isn't one of kudjoojoo bird's children,” but there's more to it than that.
Mana is inherent spiritual potential, something like a cross between potential energy, devotion, and honor, memetically speaking. Mana is generated in several ways. One such method is the natural way that it flows from mana realms into three-dimensional space, where it is translated into actions, energy, and objects. Deliberate manipulation of those actions can cause changes in the amount of mana and manifestation of it in the world. One of the ways to dramatically increase one's own mana and the power inherent in it is through acknowledgment of your own suffering by spirits (there are other ways, but most of these are highly ritualized). There are several ways to do this, and one of the fastest and easiest is chopping off parts of yourself or cutting yourself; the more it hurts, the more mana is behind your action or favor (there are even drugs to help enhance the pain and practiced torturers that spellcasters can hire, if they so choose). Normally, when someone cuts off parts of themselves, these things don't grow back, and prosthetic technology is hard to come by (though it is possible). Even so, cutting off a part and replacing it is only liable to get you the mana from just cutting it off, which is less than cutting it off and leaving it that way, causing some spirits to retroactively remove mana, getting you sick, undoing what you had done, or causing some other curse or terrible misfortune to befall the practitioner.
In the case of Ten-Ghost, she doesn't even feel pain the same way as a human does, instead feeling it as a cold pinch. It's mental suffering, in a way, and she performs rather dramatic acts to a human (chopping off a finger for a human would help pour mana into an object or item, or even start a rainstorm or cause a small earthquake, but for Lucy, all it does, with ritual songs and bribes to back it up, is help one person pass on peacefully). The mana she creates in this case isn't a lot from the pain it causes, but a good chunk from the suffering she goes through in putting up a front of socially acceptable action and behavior.
I think that covers the answer.