It's an inherent ability that improves a Lunar's shapeshifting abilities. Not all are passive, either, some give you the straight-up ability to spend motes for an effect that relates to shapeshifting (a good example is Monkey Arm Style).
No, there is no, "Use a knack to generate an immediate in-combat effect" combat category for knacks. Making one and tying it in to Lunar martial arts is what I'm talking about.
Edit:
They aren't formalized martial arts in the sense that any other martial arts are. They're the martial arts charm tree (Dexterity charm tree for Lunars) for the Exalt type in question, innate to that exalt type in precisely the way all the other charms are innate to them, that has a section that may be formalized as a traditional martial arts tree and taught to others.
There's no guarantee there.