Like form-based authentication, @CustomFormAuthenticationMechanismDefinition provides flexibility to add a custom way of authentication using J2EE technologies, such as JSF, as follows: 

 @CustomFormAuthenticationMechanismDefinition(
 loginToContinue = @LoginToContinue(loginPage = "/login.do"))
 @ApplicationScoped
 public class AppConfig 
 {
    ....
 }

We can see that this is similar to the form-based authentication, the only difference is we are specifying a custom server link that is responsible for providing form details and the underlying authentication mechanism.