It really has nothing to do with rugby. It is the haka -The Link - and now has rightfully entered the traditions of rugby. It is so much more classy than the tradition of the "rugby queen" but not essential to the actual game (just as the "rugby queen" isn't essential).
And i figure most know the haka from the New Zealand National Rugby Team, the All-Blacks, which perform the haka right before the game starts.
The All-Blacks haka is pretty awesome. The LinkThe Link Check out the 1:11 mark on the second link. Couldn't get away with that here in American sports.
Interesting side note: what is the name of the New Zealand National Basketball team? The Tall-Blacks, of course. Not joking.