Jack had been hit with several punches prior to the right which landed him outside the ring. That punch connected as well, but Firpo's follow-through was the coup de grace which was a kind of push. All said and done, I think it is fair to call it a knockdown. Jack had been nailed at least three times as I recall and was on the defensive. If his pins had been steady he probably would have maintained his balance.
I've posted since April 2007 that it was absolutely a push, a stiff armed shove which would not be counted as a KD in more modern times. Firpo planted his fist then straightened out his arm. Jack himself was pretty cool with it. "As far as I'm concerned, we're in there to raise hell and the referee's only job is to count to ten." ~ Tex Cobb.
