When someone's listening to hard rock/heavy metal reaches to an epic level, normal headbanging can't coupe with the sheer power of it. That's when windmilling comes to play.
Goes great if the person has long hair and a brain as strong an Air Force pilot's ('cause it'll gives you some fucking powerful headaches if you're not careful).
"Fuck, man! Listening to Amon Amarth, Morbid Angel and Cannibal Corpse is giving me the urge to windmill my head off!"