In the documentation, it's not clear how region monitoring is implemented. Is it purely based on significant location change, or is it a mixture of significant location change and standard location service?
From experience, I'd say it uses significant location changes until it's close enough to have to use more specific location updates, but I have no data to prove it. Only thing I'm sure of is that it works too accurate to be just significant location changes, and doesn't appear to drain battery, so I don't think it constantly uses more precise location updates.