The practice of "hardcore dancing", more commonly referred to simply as "dancing" by those that can actually do it well, is performed at many hardcore/metalcore shows. There is no "flailing" at all in dancing, there's always control in it. Dancing is performed during the breakdown of a song (you better damn well know what a breakdown is). Two-stepping is performed during two-step parts in songs, these arent too hard to find, but two-step is a bitch to learn simply because you have to find the rythm and synchronize it with the movements (two-step works in opposition; ie: right arm/left leg, left arm/right leg, got it? good).
If you still dont understand what dancing is, go to a show and watch kids that know how to dance, then you'll know it's not fighting 'invisible ninjas' or any of that shit.
If I ever hear a kid say "hardcore dancing" at show, I will kick your ass because it's not fucking 'hardcore dancing'.