In short, it's a 65.7% chance every NP that you fuck yourself up. Without the buff.
EDIT: Just to remind people, it's 30% per enemy. Assuming that waves 1 and 2 are all regular mook, that's the chance per wave that you proc IK on at least one of the three targets. Obviously, Wave 2 will have a way lower value since it'll usually never be just regular mooks but you kinda see how bad it is if you keep using her and suddenly have your nice 3-turn NP loop broken on the very first turn.