Heard about it too. But what I read is that you need 3 Missingnos and Mewtwo in your party, then you need to go to the truck by the SS Anne and use E. Coli's giga punch to flip it and defeat Mew, who will unlock the mode.
That's half the story. You also need to use your Vita to take a picture of a Mexican midget holding balloons and tweet @PlayStation a YT clip of yourself drinking breastmilk from a Cambodian immigrant. The finest. Breastmilk.
Breaaaaastmiiiilk come out and plaaeeeaaaayy --- Closet Nerd