The mullet acted as a fairing smoothing air flow between the back of the helmet and the riders leathers Murf. A bit like the speed bump on modern racing leathers. It was accepted knowledge back in the day that a well maintained 'Billy Ray Cyrus' was good for about 5-7 kph on the top end, and an early 80s Wham 'George Michael' only got you a 3-5 kph advantage.