You set the number of shards you want and click "rebalance" (CLI tools are available too, of course). The right data is replicated to the right nodes without any additional effort. It's demoed in the screencast video if you'd like to see (http://www.rethinkdb.com/screencast/).
I was actually a little worried when I read that everything can be handled from the Admin GUI interface. Knowing that there are CLI tools available makes me a little more comfortable.
For some reason reading through steps like "click here" makes me less comfortable than "type this".
RethinkDB allows users to reshard with a single click in the WebUI. All you need to provide is the number of shards you want and it will partition the data evenly in to that many shards and lay them out on machines. You can see this in action in the webcast video (http://www.rethinkdb.com/screencast/) or you can install and see for yourself.
Yes, we put an enormous amount of effort into building a rock-solid architecture. However, any software has bugs, especially early on. We're working hard to make RethinkDB production ready, but it will take a few more months until it gets sufficiently battle tested that we can recommend running it in prod. environments.
Check out their main webpage. They are better than most of the other "NoSQL" dbs out there in explaining the strengths/weaknesses of their product compared to a typical relational DB.