At the moment it's just a chance off a tick and it counts for both Venomous Bite and Windbite so you essentially have 2 chances per tick. Granted, not many people were able to experience it due to the cap being 35 and it being a level 36 trait. I expect we'll get a lot more clarification during phase 4. One thing to note is that both
Windbite and
Venomous Bite list critical hit as the cause for reset on XIVDB. From my experience, it did not reset on criticals at all prior to
River of Blood at 36. What I mean is it's not like Archer gets a double dip on Bloodletter resets.