The operational concept is quite simple, really:
1. We scan your codebase for files matching the one containing your class. This is fully configurable, but by default we use some ugly autoloader hacks to find the file without wasting disk I/O.
2. We feed your PHP file to PHP-Parser
3. We analyse the produced AST and wrap it in a matching Roave\BetterReflection\Reflection\* class instance, ready for you to consume it.
Read all about it on Marco Pivetta‘s blog: https://ocramius.github.io/blog/roave-better-reflection-v2.0/