Ok, so the obvious thing that sets it off is an opponent failing to roll high enough to hit your AC, but what about miss chance? Spells like blur, displacement, or mirror image for example? How about deflect effects like Crane Wing or a Duelist's Parry (either your own parry or an ally parrying an attack that would have hit you)? How about the effects of the In Harm's Way feat from the APG? Essentially, what exactly constitutes a miss?
nobody?
I haven't thought about it too much, but my assumption is any time an attack roll isn't high enough to hit. So, with that interpretation, Crane Wing wouldn't trigger it, for instance.
I think spells like blur make an interesting case. I would be inclined to say that those sorts of things do in fact trigger Snake Fang, since the attack is missing, whereas Crane Wing is basically a deflection of what otherwise is a hit.
I'd say a miss from concealment would count, but not a deflect/parry or "intercept" (in harm's way)...mirror image not sure but my gut says no.
Essentially foe swings and hits nothing is a miss for snake fang.