The tweak makes your device listen for the 'Ok Siri' command just like the Moto X listens for the 'Ok Google' command.
You can change the keyword from the Settings page and Activator actions can be used to toggle the feature on and off. You can also change the action from launching Siri to doing something else, if you like.
The tweak uses OpenEars for offline keyword detection. You can get it from the BigBoss Cydia repository.