It's something along the lines of a JIG for the PS3. I'm not really sure but since I've done it many times, the battery seems to have an EEPROM and a unique ID stored in it. If that ID gets flashed to a specific number in hex (I think it was 0000000F) then the PSP would boot up in "service mode" where we could boot the unsigned code and run a flasher application that flashed the CFW. So as you can see it's not the actual power source but the rom in it that was used as a vector.
PSP Go 6.60 Pro-B10
PS2 V17 FMCB + ESR
PS3 Fat 3.55 True Blue 500GB Internal + 1TB External
iPod touch 4 4.3.3 Jailbroken
Xperia Arc MIUI 1.11.25