In the spec, there is following code sample explaining how to achieve specialization:

@Alternative @Specializes
public class MockAsynchronousService extends AsynchronousService {
... 
}

This is IMHO misleading as the @Alternative annotation is not needed for specialization to work and hence should be removed from this sample. In fact one user approached me recently, asking me some questions regarding CDI specialization, and this sample was a source of his confusion.