North Korea’s Mightiest Weapons? Nukes and Shirtless Men on Broken Glass.

But it’s North Korea’s military stunts that often earn attention on the internet. The demonstrations include a mix of taekwondo​ and performances by “charyeoksa,” the word used to describe itinerant Koreans who used to roam the countryside as part of a traveling circus, showing off their superhuman strength​.

The old circus performers attracted crowds through their perilous feats, such as driving nails into wood with a head butt or bending a steel rod with ​their bare neck, as the North Korean soldiers did for Mr. Kim on Monday.

During the Cold War, when the militaries of South Korea and North Korea had few sophisticated weapons but plenty of mutual enmity, both countries would stage martial-art demonstrations​ as morale boosters. South Korea has mostly abandoned the demonstrations as its military modernized, occasionally staging them during Armed Forces Day.

In the North, ​kick-and-smash displays of military power remain a favorite genre of propaganda.

​Mr. Kim was so proud of his military stunt squads that he invited China’s leader, Xi Jinping, and his wife to a performance when the couple visited Pyongyang in 2019. When the North wanted to show its displeasure in 2012 after the former South Korean president Lee Myung-bak stopped inter-Korean trade in response to the North’s nuclear weapons development, it released footage of commandos throwing hand axes and knives at Mr. Lee’s name.

These demonstrations are usually performed by members of the elite units assigned to protect Pyongyang, ​Mr. Kim and his safe houses and villas around the country. They are called “human rifles and bombs” for the North Korean leader. The same unit of soldiers arrested Jang Song-thaek, Mr. Kim’s uncle, when he was executed on charges of treason and corruption in 2013.

“It is said in the North that each of these soldiers is trained to fight and beat as many as 10 men at the same time in a hand-to-hand battle,” said An Chan-il, a North Korean defector. ​But like North Korean children mobilized to perform countless hours of practice for the Arirang Mass Games, the North Korean soldiers behind these stunts put themselves at great risk.

Choi Won-young, a taekwondo master in South Korea, broke a stack of granite slabs with a head butt during Korea’s Got Talent, a television program, in 2019. During the show, he warned the audience not to try it at home: “When the slabs are not broken through, you blackout and then regain consciousness.”