Basically a variation on skanking. Which was a dance you did while
listening to ska or punk rock music. Skanking is usually done in a
circle pit. 2-stepping is done usually during a breakdown of some hella brutal metal music. You don't go around the pit or anything. You usually just stay in once place and 2-step that shit.
Hardcore dancing is sometimes referred to as "hardcore 2-stepping" or "slam dancing".