For the runtime, there is not much dark magic. Each type that has an `extend T::Sig` has `method_added` hooks registered, which notifies Sorbet runtime whenever

For the runtime, there is not much dark magic. Each type that has an `extend T::Sig` has `method_added` hooks registered, which notifies Sorbet runtime whenever a method is defined on the type. When that `method_added` hook is called, Sorbet runtime uses the sig flag that you mention to associate the `sig` with the method definition that follows it. It is, more or less, an implementation of this idea: https://yehudakatz.com/2009/07/11/python-decorators-in-ruby/ https://yehudakatz.com/2009/07/11/python-decorators-in-ruby/