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).
Windmill headbanging, harder to do but much more fun to watch than normal headbanging.
"Fuck, man! Listening to Amon Amarth, Morbid Angel and Cannibal Corpse is giving me the urge to windmill my head off!"
At an concert or arena, someone standing behind you
accidentally or purposely places their junk on your head.
Dan was at the arena, Winston standing behind him talking to Carl. After several minutes, Dan realized that Winston's junk was resting comfortably on Dan' head. Needless to day, Dan was unimpressed. Winston headbagged Dan. Prime example of a headbagging!
When one is sleeping in class, and his/her headbobs up and down at random, providing a source of distraction and entertainment for everyone else in said class.
Hey, did you see that kid slow-motion headbanging in Math class today? What an idiot.