Looking at Rubio and Murray after they were hit on top of the head, they had nothing left. I don't remember anyone using it like that either.
00:28 [yt]C_NyqeNLheQ[/yt] AFAIK its legal and very effective but it is very open to counter so you cant use it unless you are confident that your opponent is not in a position to counter. It is effective because you hit your opponent where he is not used to get hit, like a punch behind the ear(Price vs Thompson: 1 punch KO), so it will give you an unfamiliar kind of sensation which by reflex you will tend to go down and get knocked down/out.
If you look at someone from behind basically you can't hit what you can see there. Anything you can see from the front you can hit. If you look at someones head from above (birds eye view) : Anything in front of the ears you can hit. Anything behind the ears is the back of the head therefore can't be hit The most common illegal punch which people get away with is behind and above the ear.
it is borderline...but it wasn't thrown with the intention of landing in an illegal place..a legit KO. Looks like it lands on the ear & that's perfectly fine. Matthysse Molina 2nd knockdown that's an illegal punch...wouldn't say Molina did it purposefully though.
I think that is a good and legal punch. It is designed to be a surprise and to penetrated the borderline between the forehead and the skull against a high guard. Future opponents will have to chose between protecting the chin or against that punch.
Do not cover up and bend down, exposing only the top of your dome and GGG will not accordion your neck for your troubles. Simple as that.