There have been hacks around for some time to power the GoPro through the USB port or the expansion port. Now, you can power a GoPro through the expansion port while getting live video out through the same port.
The main advantage to external power is reducing weight of the GoPro and eliminating one more battery level to monitor during FPV flights.
I picked up this small board from ReadyMadeRC. After soldering 3 wires on and hooking it up to my TBS Core OSD, it worked! I tried it with both my GoPro HD Hero and Hero2.