Pull in magic and shape it

But more important, once that pentagram came up-and it was coming now; I could feel it, a faint stirring of power that slid along the edges of my wizard's senses like some huge and hungry snake passing by in the darkness-it was going to shut the building off from the rest of the world, magically speaking. That meant that I wasn't going to be able to draw in any power to use to defend myself, any more than I'd be able to breathe if someone plunged my head underwater.
Usually, when you work a spell, you reach out into the environment around you and pull in energy. It flows in from everywhere, from the fabric of life in the whole planet. You don't create a "hole" in the field of energy we call "magic." It all pours in together, levels out instantly, all across the world. But the circle about to go up was going to change that. The relatively tiny area inside the Shedd would contain only so much energy. Granted, it would be a fairly rich spot-there was a lot of life in the building, and it had hosted a lot of visitors generating a lot of emotions, especially the energy given off by all those children. But even so, it was a sealed box, and given the number of people present who knew how to use magic, the local supply wasn't going to last long.
Try to imagine a knife fight in an airtight phone booth-lots of heavy breathing and exertion, but not for long.

Small Favor Chapter 30, Page 248