I don't think that he is doing it under pressure. I think he's just opening the valve on the fermenter from an elevated position above the keg and the goodness flows down to the keg, meanwhile the co2 in the keg is being displaced by this goodness and pushed back up into the bucket.
Hmmm interesting. So he fills an empty keg with co2 and I assume the beer coming out of the bucket through the spigot goes into the dip tube (which is beer out) and the co2 being pushed up from the beer filling in the keg goes out the gas in post into the airlock.