The extended_on, deviates_on, and not_supported_on are all guards for alternative implementations. They do not apply to the Standard Ruby implementation (aka MRI 1.8/1.9). They will raise an exception if :ruby is passed. They will also not yield if a platform identifier like :windows is passed.
If mspec is passed a :some_string type argument, it will attempt to look up a config value, which can be a single value or an array, and attempt to process it for spec files to run. This simplifies running e.g. all the library specs now that the ones for 1.8 and 1.9 are colocated.