when you knock a guy out by just throwing one punch not like where you make them go in to defense mode but its just like a flash knockdown is there a way you can always do that like a code or something? or is it just like a lucky punch?

With me, as many times as I played that game I only performed 1 flash KO.
That was during online play.

You have to throw a haymaker while your opponent is leaning into it or say you block a punch and you wind up and hit clean he will go down.