If you look carefully Hearns has his elbow way down and his hand way up as if waving to duran at high speeds.
It catches duran on the chin and knocks him out.
How did it have any power without proper bone alignment?
Was it a mistake or is this an advanced pro punch?
I have a much better slo mo gif from a better angle but I cant upload it, this is next best, it looks like a straight right at fast speed but I assure you his arm is bent quite a bit at the elbow and he strikes with the bottom of his fist