If they slapped you hard at the wrong part of your face, you will be prone to a very serious brain damage or injury. Which is why for me, this is somewhat a dangerous sport to explore with. Watching this is fun but I don't think it should be included in the official sports list. Betting on this may only be available on their local bookies only. But one thing I can say, this is truly an entertaining activity, without of course potential injury from both players.
Yes, it could seriously cause a great injury if the slapper could hit the prone part of your faces like the ear and other parts that are crucial to be hit with a slap, and even though there are rules in disqualification that they can not hit the Ear, and parts that are prone to damage, wrong calculation or accidents can surely occur if the slapper is not accurate with his hit,
I also think that this kind of thing will only be available in their local place for entertainment and can not be made as an official sport activity.
Maybe there are requirements for people who want to join in that competition or tournament because it can get them in danger. Betting on the person with the huge muscle will not always win us because I think a little guy does not seem too strong but can beat his opponent. Maybe that will be only popular in that country and their people called that game a sport.
I don't really know if there are requirements to this kind of thing because I can see different kinds of people that are participating, Maybe there are medical examinations before doing the slapping match or there are certain division like in the UFC to differentiate their weight class but one thing is for sure this thing is very popular on their country.