If someone asks me how Rails works, I wouldn't be able to answer. It's just something I don't care about. Sure, give me a few hours and I'll figure out everything you asked for, but right off the bat? No way.
ActiveRecord is the most obvious example of this. If you don't know what queries it's going to throw out for a particular call, it may work just fine in development but could easily bring down a site in seconds in production.
Routing is another example. If you have a particular action that needs to have as low latency as possible changing its position in the routing file is often one of the easiest and most effectual changes you can make. Someone who just codes in the framework and doesn't understand how it works is unlikely to realize that.