Had a couple clarifying questions re: the Puca trait of being unable to break promises.

First, is it that they're outright unable to willingly break promises, or can they do so with great effort and just suffer consequences for it? Second, what happens in the case of an outside circumstance making a promise unfulfillable or conflicting promises?

Like, I assume that as with all powers there's a fair amount of "up to the player's discretion" involved, but figured it'd be good to ask for a general idea on these.