Mashing the Denjin

Well, I see people do this weird mashing after activating the Denjin Hadoken but I was wondering how exactly this is done? When I attempt to do such a thing, the Denjin just gets released… only doing one hit.

Hold the punch button you used for activation until you want to actually release the Denjin. You can mash on the other buttons if you want, but I usually just wiggle the stick back and forth (some people prefer a circular motion).