Interestingly this recently cropped up in the sim ( unrelated ) and - as stated - the gangbar shuts down everything and takes about 30 seconds for everything to reboot during which time the aircraft could do anything. In this case the aircraft was recoverable but was in a very unusual attitude when the lights came back on.............. Turning everything off ON THE GROUND was an unofficial engineering CTR ALT DEL to clear snags otherwise requirering a shutdown etc etc. It was never recommended to crews and certainly not in flight...