How does compass calibration work?

I am wondering how compass calibration works / why e.g. drawing an 8 or rotating the phone to all 3 axis works (or at least shall work) - I mean I would understand the calibration process, if an application would say me: Point to north, then press that button down there, but you can start at basically zero knowledge for the app, just by rotating your phone! How is that done? Also, do I need to enter a special calibration mode or can I draw these figures (like the 8) directly in Google Maps? How does Google Maps know that I am not just really moving the phone but want to calibrate the compass? Thanks for any hint!