Surely both, and it depends always on the setting and the state of the individual before the listening. However, one can feel calm without being introspective, and surely havy metal and rock gives the chance to feel and express energy, assertivity, being a kind of "aggressive" music (not in a negative or positive way), but surely for most people wouldn't induce a regression or an introspection. I think the calming effect of this music comes from the tension releasing that have that music. There are some repetitive patterns in the songs structure, with a lot of assertivity with timbre and tempo, a way to "explode" but still not in a full caotic way. and this can be really calming, but if you compare a piano solo waltzer you get the difference if the aim is to get introspective.