As stated the arduino auto-voltage switch will make external Vin voltage override USB power. Your concern and desire to understand this concept is important, because the larger question is what is your Arduino trying to drive external it's internal needs? The USB power is limited to 500ma max, so it's your total system current requirements that determine how you are going to have to set-up and wire your project. Also how to power off your system if plugged into both usb and external power connector should be understood and accounted for.
Lefty