Wondering about Apple Magsafe Air Battery

I’m thinking about buying the magsafe battery, but I still wondering this: When the battery is attached to the phone, I’m not sure if the phone’s main power supply is coming directly from the MagSafe battery (meaning it bypasses the internal battery) or if the phone uses its internal battery while the MagSafe battery charges it continuously to maintain the percentage. Do you guys have any sources that have seriously tested this out?