When you put your ballsack on your sleeping mate's eyes to half cover them and then shine a torch down onto your balls. It gives the recipient a blurry image of the torch light, resembling a car's headlights on a foggy night.
Mark: "Hey man, Jay's sleeping on the lounge, do something to him"
Simon: "Pass me the torch, I'll do the Hungarian Headlights"
Mark: Ew dude, that's gross.