When the meaty palm of Vasil Kamotskii, a 360-pound, 34-year-old pig farmer from Siberia known as Dumpling, struck the tender cheek of the man who faced him, it sounded like a thunderclap. Dumpling didn’t appear to expend much effort — he swung lazily, the way you might bat a fly. But it was enough to send his opponent, Kamil Marusarz, a 26-year-old from Orland Park, Ill., toppling to the ground.

Referees and the medical staff onstage at the Cobalt Ballroom at the Fontainebleau Hotel and Casino in Las Vegas late last month rushed to check on Mr. Marusarz. Any hesitation in the cheering and applause from the 3,500-person crowd was alleviated when someone with a clear view of the ring yelled out that Mr. Marusarz was still breathing. The atmosphere had the boozy jocularity of a bachelor party.

Dumpling raised a fist, grinning in triumph as the announcer declared him the victor. Mr. Marusarz remained unmoving on the floor. The whole fight, if you can call it as much, lasted about 30 seconds. Most fans agreed that it was the highlight of the evening’s eight-bouts event, in which one pair of competitors after another stood their ground and exchanged earsplitting slaps.

Dumpling has been slap fighting in his native Russia for many years — in fact, he is considered one of the forefathers of this unlikely pastime, helping to popularize it with viral highlight videos on YouTube — but this was his first time participating in Power Slap, the big-money slap-fighting league created by the U.F.C. president Dana White. Mr. White was inspired to found the league after happening upon Dumpling’s videos in 2021. Impressed by the attention-grabbing conceit, he wanted to see what would happen if the sport were “done the right way,” which is to say by him.

“The answer is seven billion views in 17 months, that’s the answer,” Mr. White, 54, said in a recent interview, referring to a statistic he often mentions about the Power Slap league’s total number of views across multiple social media channels, including YouTube, TikTok and Snapchat. Mr. White likes to rattle off follower tallies and how they compare (always favorably) to various major-league sports. “Power Slap has gained over a million and a half Instagram followers just in 2024 so far,” he said. “Which is more than NASCAR, Major League Soccer, the N.F.L., Major League Baseball and the N.H.L.”

Extreme Risks

Slap fighting was once a shapeless contest of trading hard, openhanded blows until knockout. Mr. White, with his considerable resources, has ushered it toward, if not respectability, at least a semblance of order, establishing a set of official rules and putting in place protocols that give the enterprise the appearance of legitimate sport. To that end, Mr. White said that the organization had “run toward regulation,” actively working with groups like the Nevada Athletic Commission, which officially licensed the organization to host events under its jurisdiction, to bolster the league’s legitimacy and to “make sure that the sport is regulated and safe.”

But Power Slap’s path to legitimacy may be even steeper. Skeptics say that regulations are basically irrelevant — that damage caused by slap fighting is intrinsic to the action and can’t be mitigated.

“This is not a sport, OK? This is an event,” said Dr. Gregory O’Shanick, medical director at the Brain Injury Association of America. “A sport is a contest of athleticism or skill. This is merely your physiological ability to withstand blunt-force trauma to the head. It’s like seeing how many times somebody can run into a brick wall.”

Mr. White pointed to the U.F.C., another brutal combat sport, as a precedent. “We don’t get enough credit for this: Never a death or serious injury in 30 years of the U.F.C.,” he said. “It’s a combat sport, but we spend the money to make sure it’s as safe as you can possibly make it, and it’s the same with Power Slap.” (Some studies have indicated that U.F.C. fighting has also potentially caused traumatic brain injuries.)

Frank Lamicella, the president of Power Slap, had a more laissez-faire attitude. “Look, there’s two people hitting each other in the head, and if I was a doctor, I’d probably tell them, ‘Hey, this is maybe not the best idea,’” he said. “But if two people want to do it, we provide the platform, and we spend a ton of money making sure they’re safe.”

Mr. Lamicella, who used to be a lawyer at the firm Paul Weiss, and became close with Mr. White and the rest of the U.F.C. team after helping to facilitate the deal that brought the U.F.C. under the umbrella of Endeavor, expounds upon the rigorous so-called safety protocols the league has established. The competitors get M.R.I.s, M.R.A.s and electrocardiograms; physicals, blood exams and eye tests. There are two referees, “more than in any other combat sport,” and multiple teams of emergency medical personnel and ambulances on standby, in case a competitor needs to be taken to the hospital.

Dr. O’Shanick is among the authors of an open letter, released by the Brain Injury Association of America, calling for Power Slap to be banned. The blows sustained in a slap fight, he said, are worse than what you might experience in football or boxing, largely because competitors are not allowed to defend themselves, meaning they are likely to be struck on the head multiple times in the same place over the course of one fight. That can lead to concussions, hearing loss, seizures or even chronic traumatic encephalopathy. “There will be deaths from this,” Dr. O’Shanick said. “There is no doubt. There will be deaths.”

Dayne Viernes, a slap fighter known professionally as Da Crazy Hawaiian, expressed initial reservations about the safety of the sport, in part because of warnings he had received. “There was a lot of people telling me about C.T.E. and brain damage and all this stuff,” he said. He has come to accept the risk, however. “Right now I’m listening to my body, and I hear it very well, and I don’t really believe that it will affect me the way some people think.”

Dr. O’Shanick said that because of the nature of these kinds of brain injuries, sufferers of C.T.E. do not always know they are affected.

Critics have been alarmed by the possibility that these behaviors might be replicated on the schoolyard by impressionable children, particularly given the sport’s growth across social media.

Dr. O’Shanick said that he worried about youngsters doing this, but as adults go, he seemed more flippant. “If you take a libertarian perspective, you don’t legislate it, you just let natural selection happen,” he said. “Eventually all the people engaged in this are going to die out, and you’re not going to have to worry about that population reproducing.”

Though its Las Vegas events stream live on the video platform Rumble, most people are watching Power Slap on social media, where they have the opportunity to offer feedback in real time. The tenor of the comments on Power Slap videos tends to be somewhat apocalyptic: People aren’t just saying that it’s stupid, but that it’s somehow emblematic of the stupidity of mankind.

“It’s from a slap — just a slap,” Mr. White said, laughing. “They’re acting like you just watched somebody get hit with a baseball bat.”

Mr. Lamicella said that the deluge of negative commentary online had actually accelerated the growth of the sport, because social media algorithms favor the volume of response without distinguishing between positive and negative replies. “Whenever you leave a bad comment on a Power Slap video, that helps me, so thank you,” he said. Power Slap now lists over 80 strikers on its website.

But Mr. White (who was caught slapping his own wife in public last year) said that he had experienced this kind of criticism before, when he was attempting to grow the U.F.C. “Everyone said that the U.F.C. wasn’t a real sport, that it was barbaric, that it was never going to work,” Mr. White said. Since its founding in 1993 and especially since Mr. White took over the organization in 2001, the U.F.C. has matured from niche to mainstream, with over $1 billion in annual revenue and millions tuning into its regular pay-per-view broadcasts on ESPN.

At the end of the night at the Cobalt Ballroom in Las Vegas, as Da Crazy Hawaiian was celebrating his victory in the headlining super heavyweight championship fight, he smiled and roared, lapping up the cheers of the crowd. As he thrust his championship belt into the air, an announcer asked him if next time he would be willing to take on Dumpling. “I want Dumpling so bad,” he yelled, giggling and rubbing his belly. “Feed me! Feed me!”

When asked to explain what he would want the world to know about Power Slap, if he could let them know one thing, Da Crazy Hawaiian paused for emphasis. “Like it or not, this is a sport,” he said. “And whether they like it or not, we’re doing it anyway.”