This document explains our basic replication architecture and might be helpful http://www.rethinkdb.com/docs/architecture/

Let me know if you want to know more about a specific aspect.

So, I am right in reading that there's no automatic failover? Also, when a new master is chosen, do you pick the replica with the most up to date log or do you hand that decision off to the administrator?

Sounds like you need to implement raft. Do you have any plans in that direction?