Circle rules

The other part of magic you need to know is magic-circle theory. Most magic involves a circle of one kind or another. Drawing a circle sets a local limit on what a wizard is trying to do. It helps him refine his magic, focus and direct it more clearly. It does this by creating a sort of screen, defined by the perimeter of the circle, that keeps random magical energy from going past it, containing it within the circle so that it can be used. To make a circle, you draw it out on the ground, or close hands with a bunch of people, or walk about spreading incense, or any of a number of other methods, while focusing on your purpose in drawing it. Then, you invest it with a little spark of energy to close the circuit, and it’s ready.
One other thing such a circle does: It keeps magical creatures, like faeries, or even demons, from getting past it. Neat, huh? Usually, this is used to keep them out. It’s a bit trickier to set up a circle to keep them in. That’s where the blood comes into play. With blood comes power. If you take in some of someone else’s blood, there is a metaphysical significance to it, a sort of energy. It’s minuscule if you aren’t really trying to get energy that way (the way vampires do), but it’s enough to close a circle.

Storm Front Chapter 13, Page 154