Using spirits a connection is forged between the object that is similar in appearances, surroundings, and number, which can then be manipulated and influenced by the user. An expert sympath can form a connection between a doll and a person, and inflict harm on the individual by harming the doll, which is the basis of Voodoo.
This even extends between realms, such as the mirror realm and the physical realm. It also explains why damage done to buildings in the Spirit World will be reflected in the physical realm over time.
There is a weakness to the technique when used from the mirror realm, however. Since the items in the physical world have more "weight" then if the items used are move then the reflections react in the same way.