However, as long as I was channeling ray of frost, it seems like stuff would die soooo fast in a chain once the puddles started happening. So I think what happens is that black ice gets triggered on assists. That means that if you are kind of strafing around with the beam, then anything hit by the beam in the last little bit (1 sec? half sec? who knows) triggers a new puddle.
Long story short, though, is that arcane dynamo with black ice is maybe the fastest killing anyone in the entire game can possibly do. Give it a shot. As long as you are fighting multiple mobs, you will kill so fast that your head spins.