Johnny M pretty much nailed it there.........
There's other examples, like the Anadin Sisters dancing in She Sells Sanctuary vid by the Cult. Think they were in a Doctor n The Medics vid aswell? Lots of Goth/Alternatives bands had similar things, but NMA's following/Militia/family call it what you like took it on and it morphed into what it it became by the late 80's.......... Some of us dinosaurs still do it alot, some less......... I think it's made a bit of a resurgeance in the last 5 years........ I'm glad to be honest, it's our thing so to speak. I hate hearing the pit called a MOSH PIT to be honest. Mosh Pits are a hardcore punk thing, but maybe that's coz I'm firmly rooted in the hardcore cene as much as NMA. So the differences are more pronounced to me? Dunno.........