content: Hijacking top comment to post a much better explanation from from Matthew Prince of Cloudflare at Black Hat USA. It's an hour long, but really interesting to watch and they show how they mitigated an attack twice as big as the OP video. It's probably not how Prolexic does it, but they dont explain shit in their video. TL;DR: they use anycast protocol. A bunch of servers all over the world are ready to accept the traffic for a single destination IP so it dilutes the attack across the network.