I was going to say what others already said, how absolutely idiotic this slapping is. Brain damage, ear damage, maybe even neck damage? And for what.
A while back I read a former kick boxing world champion turned brain-protector (I don't know what to call him lol) who is now adamant that even fighters should accept zero head shots in training, and it should absolutely be completely banned for underage fighters who can't form informed consent due to immaturity. He says it's not like it has to be just cumulative damage that you should look out for, brain damage appears from the first hit. He claims he can often see signs of brain damage from the very first shot in bouts.