The only way I could "rock out" to Sigur Ros is if I had a few extra drinks...and my stationary stance became a little boggled - which would translate into an unintentional yet extremely dizzied groove.
Most the rocking was because the band's drummer is really fucking intense on stage. At one point, he was playing so hard he broke part of his kit. Rather than stop, he just played with what was still standing, while a roadie did repairs.