You probably shouldn't believe everything you see on the intarweb, some is false, some is true. And I'm not sure I would call it a phenomenon...you may not be doing it right eh?
We aren't talking about girls who make the sheets wet afterwards; those are definitely real. We're talking about the girls who can apparently fuel a slip and slide for a few hours. I honestly refuse to believe there is a water park housed inside any woman's vagina.