If you get right down to it, spirit magic is not really "magic". It is the natural way that the spirit world with the hard world. Nothing incomprihenciple or unnatural about it. The same applies to the Spirit world itself: it has natural laws that it follows. They are just different from the rules in the hard world.
Who?
Spirit magic can only be practiced by spirits. When a priest, a shaman or a monk practices spirit magic it is actually his spiritual part doing the deed. Practicing spirit magic an effort of years. It is, after all, pretty much equal to the process that children go through when they are born. Suddenly they pop into a world where nothing makes sense any more. They can't speak, they can't walk, they can't even really see anything. However, with proper teaching and lot's of practice one is able to learn to do these things in the spirit world.
How?
The spirits interact with their surroundings by projecting their thoughts on them with their will. This projection is a natural skill that comes to spirits, and they become better at it, with experience. The surroundings may be just generic spirit stuff, but they may also be other spirits. If the target is a bonded spirit, this manipulation will effect the hardworld counterpart of the spirit also. He will start to feel differently, or strange thoughts will pop up in his mind out of nowhere. If the manipulation is very powerful, the person may become possessed by the manipulating spirit.
An experienced spirit notices immediately if someone is trying to focus their will on them. They can fight back by enforcing their form by will, or they may flee from the place - a spirit can only influence spiritstuff that is near it.
Sharing
Sharing, or reincarnation, is a special kind of possession. At the time of birth a powerful spirit may share the body of the child. He possesses the child but at the same time surrendures himself entirely to the will of the child. The two entities - their thoughts, minds and fates - become totally intermingled. At first the mind of the child will dominate the body, but when reaching adolescence he will start to remember things that belonged to the spirit. Upon entering adulthood he, or she, will have all the memories intact.
Sharing is a dangerous business for the Immortal. His fate and that of the child blend together and unless he is careful it may be that he will truely die along with the body and never become an Immortal again. Also, he is quite helpless for the first 15-20 years of his new life.
Aspects
It is possible for the spirits to "copy" themselves, in a limited sense. By manipulating a mass of spirit stuff for long enough, they may imprint some of their own properties on them. These ascepts can then act more or less consciously. This is very hard and arduous task and only the most powerful spirits are able to create aspects
