Literally just what I said. Someone claiming to be Felix was shitposting on 4chan, so when someone said "Alright, slick, if you're really Pewdiepie then turn one of the plushies in the background of your next video around and upside down" and he did it. They named a specific plush and exactly how it would be manipulated to show he was telling the truth, and Felix did do it. Proving that one of the most influential youtubers of the time was, in fact, a chan member and hadn't forgotten their internet roots.