It resets OpenFirmware. The bootrom is read only, except to Apple's special tools. Apple has never released any bootrom updates for a G5, thus there is no way to reset or write a bootrom on a G5.
They did actually. http://support.apple.com/downloads/Power_Mac_G5_Uniprocessor_5_1_5_f2_Firmware_Update
It contains BootROM file and flasher.
BUT...
flasher must be hacked to work on other boards than it was programmed for
one should extract BootROM file from DP board Apple didn't release update for that one
Last, crucial thing: what the hell for?