I'll dig in the source code as I'm very interested in how it's implemented.
Essentially intelligent sharding (in &bang's case, by team). Each node.js (or other processes) can look-up which Redis server-set owns a team, and can probably just stay connected to most all of the Redis instances. For HA, slave each shard, AOB, and off-server backups every 15m. Along with a gossip protocol for giving up on masters and promoting slaves.
According to our tests, we don't have to worry about it for awhile, but we've got a plan regardless and will start implementing it soon.