Oh, Magikarp. Poor, sad Magikarp. The most useless of useless Pokemon, training it until it evolves into Gyarados can be a brutal chore, since it can't do anything of use in battle. With the useless Splash as its go-to move, players are left wishing for something better--one good, brutal attack that blew away its unsuspecting enemies. Man, wouldn't that be nice… so nice that some people really started to believe in it. Poor, sad suckers.
This rumor, like all things involving Magikarp, started off as a joke. Some players thought it would be hilarious if Splash was secretly a one-hit kill with a low chance of success, because how epic would it be to take down a legendary enemy with the weakest Pokemon ever? Eventually humor became consideration became belief became rumor, and now there are players who swear by this rumor's legitimacy. Sadly, a joke is a joke, and no such Splash damage boost exists. Too bad--imagine a Magikarp beating a Mewtwo. The stuff of legends.