So 6.6v is the cut off point for Vin to power the device.
Not really a cut off point, as the board will still be powered by whatever the on-board +5vdc regulator is putting out even if Vin is below the 'input regulation drop-out' spec of the regulator. So Vin is never cut in or cut out, just USB is cut in or cut out if Vin is below or above 6.6vdc respectively. The FET circuit switching action is SPST not SPDT.
Lefty