We've gotten it once on S1 (Only takes about 5 min to kill Nine tails). Gave an extra 30 coupons to whoever made it breathless and seemed to make Nine tails stop attacking for a short period. It looked like the person who made the hit to make Nine tails at half hp got the reward. Seeing as it's only happened once in our server so far it's probably just an extremely small chance for it to happen each time.