The competitors stand rigidly upright with their hands behind their backs, waiting to absorb a brutal slap to the face.

When the open-handed blow is delivered, there’s a sharp report and the reaction can be dramatic. Some fighters barely move, while others stumble backward or fall to the floor. Some are knocked out.

UFC President Dana White is selling slap fighting as the next big thing in combat sports, putting his money and the resources of one of the world’s foremost mixed martial arts organizations behind the Power Slap League. The Nevada Athletic Commission has sanctioned the league for competitions in Las Vegas.

Some slap-fighting beatdowns have gone viral, including a video from eastern Europe showing a man who continues to compete even as half of his face swells to seemingly twice its size. Such exposure has led to questions about the safety of slap fighting, particularly the risk of chronic traumatic encephalopathy, or CTE, a degenerative brain disease believed to be caused by repeated blows to the head. A former chairman of the commission, which regulates combat sports in Nevada, says approving the league was a mistake.