It is a considerable change to the established Clausewitz event system, that apparently originated from the development of Crusader Kings II. Johan later explained on the forum for that game that:
The idea about this is to have a system where you can keep adding events to the flavour pulses which will just ADD to the variation and fun of the game, without eventually choking the player with event-spam.
Pulse events don't work with the traditional mean time to happen. Instead, the candidate events and their base chance factor are defined in common\on_action.txt, and the MTTH section in the event script modifies that base chance.
This means that while this section is still called 'MTTH', numbers work the other way around, with high modifiers increasing the likelihood of an event instead of increasing the average time before it will fire.