Two things to know before hitting play on this clip of Bieber’s first live “Boyfriend” outing: (1) It involves Bieber pelvic thrusts — several of them, all synchronized to extra-loud drum bursts, just in case you’d glanced away or started Googling One Direction or whatever, and (2) the best, most confident part of the performance is … the rapping? By which we mean: Yes, Justin did in fact sound a little rocky on the vocals last night, especially when it came to the falsetto (he didn’t even attempt it, really). It’s not like Bieber was ever a concert hall staple, we know, but the growing pains are showing here when it comes to range. And dancing-wise, Biebz is still working that stand and point — though he does deserve some credit for a decent Beyoncé shoulder-shake. One last question: Has Justin always done that wide-stance waddle, or is that a new Swaggy move? Please advise.