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.