It would be an increased probability, not completely automatic, but keep in mind that as a Talent, players would be unable to turn off "Punchable Face". So in every single battle, they would take a lot more of the attention, even if you would sometimes rather that they don't, so it comes with an inherent drawback (hence why its secondary effect is slight armor regeneration).
Another suggestion, other than the one you provided, would be to make it turn off at 50% HP, similar to how "what a rush" is inactive until the 50% mark.
