There is no trick, it's just basic metaprogramming.
Suppose that you have a simple event system, with annotated listeners receiving Event objects. To process something like this at runtime, you need to ask every class to subscribe to the bus, then iterate all of its methods to find the annotated ones to call.
That works, of course, but it's not that fast. Ah, if only you knew in advance, such as at compile time, who's going to get called with what... oh, wait, you do.
