What does force_ssl do in Rails?

In a previous [question][1] I found out that I should be setting nginx ssl termination and not having Rails process encrypted data. Then why does the following exist? config.force_ssl = true I see this commented out in the production config file. But if the expectation is that nginx will handle all the ssl stuff so that my rails app doesn't deal with encrypted data then what does `config.force_ssl = true` do? Should I leave it commented out in production if I know I will always be using nginx? [1]: https://stackoverflow.com/questions/15673328/how-does-nginx-pass-ssl-encrypted-data-to-a-rails-app