Why is it that guys dance in place (especially when seated) with that forward almost-autistic to-and-fro rocking movement and think that they are jammin? I mean the whole 80s head-banging heavy metal hair band genre alone should suffice as evidence to this dance craze sensation.

It is a pseudo sexual movement, I'm sure. That's probably why dancing is considered sinful in some religions. And I don't think that it's necessarily that the intention of the dancer is to be risque - it's just that's how white boyz dance.