Yes, it lets you implement event loop for Ruby. We use fibers to avoid callback hell and avoid rewriting existing code. This is covered in my talk here: https://www.youtube.com/watch?v=qKQcUDEo-ZI
Hope it's alright to ask you a personal question: How did you get to the point you can work with Ruby internals so well? Did you come from a C background or had some backgrounds with VM's / programming language design?