The novelty is that routing is based on cryptographic identity. Yggdrasil's IPv6 addresses are actually truncated representations of public keys. You configure the Yggdrasil software with a list of peers which it connects to over normal internet, but then when you route a Yggdrasil address your device talks to all its peers, who talk to their peers and so on until they find your destination. As I understand it, they optimize it by caching the routing information and using bloom filters to find the appropriate peer.