How does a notification get triggered?

Please bare with me as I am still attempting to get a better grasp on how Muse works.  As I understand, a notificationproducer capability notifies existing subscriptionManager capabilities, which in turn send notification messages to Notification consumers (which maybe outside of muse).  My question is, how does a notificationproducer get triggered or trigger itself to send that notification to the subscriptionmanagers?

Thanks,

msw