Hmmm, I don't remember exactly what I did. I copied the Derpy layer into a new layer and gave it a blur filter, but I don't remember which. Then I think I erased the blurred areas on her front to leave just the ones on her back. Hope that helps!
A Derphole is arguably 20% cooler than a Sonic Rainboom, and she manages to do what even the CERN supercollider can't do yet. (Why can't she aim herself to punch a portal into our reality, so we fans can all leap through and take a little holiday in Equestria? It's so much more fun than Vegas, plus no Celine Dion to put up with!)