Chagrin:
http://arduino-info.wikispaces.com/arduinopower explains everything.When the relay is unenergized there is continuity between A and B. When you energize the relay it switches to provide continuity between B and C.
I've already read that page, but the module they use is different, so it is not clear if the wirings will be valid for mine's. I'm interested in the "Normally Open" case.