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Professional surfer Koa Smith is gearing up for the Volcom Pipe Pro surf competition on the North Shore of Oahu. The beach is packed with fans, and the waves are thunderous. Smith is lying on his back on the floor of a house abutting the sand and breathing very deliberately, trying to keep it together. “I’ve always had a hard time with nerves before contests,” Smith says. “I’d forget how to surf. But now I breathe and it scrapes away all the bullshit.”

Smith is one of a growing number of athletes—from and his wife, , to CrossFit coach Brian Mackenzie and biohacker Tim Ferris—who have begun using the breathing exercises of adventurer Wim Hof as a way to achieve peak physical performance.

The 57-year-old Dutchman is an unlikely fitness guru. Hof has spent decades pursuing obscure feats, nearly all of which are designed to demonstrate his singular ability to withstand extreme cold: climbing to 20,000 feet on Mount Everest while wearing only shorts and shoes; running a marathon barefoot above the Arctic Circle; staying submerged in an ice bath for nearly two hours. Dutch TV has dubbed him the Iceman.

Hof credits much of his success to his breathing exercises, a practice he began five years ago. And he says his technique—which involves a series of deep, rhythmic inhales and exhales, followed by breath holding—can strengthen the body, improve the immune system and circulation, prevent disease, and help with focus, confidence, and mindfulness. Surprisingly, research backs up many of those brash assertions.

In 2014, showed that people could learn to control their immune response and autonomous nervous system after just ten days of Hof’s breathing exercises, meditation, and repeated exposure to cold. In the study, 24 participants—half of them trained by Hof—were injected with the endotoxin E. coli. Those Hof trained had a different inflammatory, immune, and hormone response, allowing them to fight it off significantly better than the other group.

“If Wim told me that in person, I would have said, ‘This is hippie yoga shit,’ ” says Andy Galpin, a researcher at the Center for Sport Performance at California State University at Fullerton. “But PNAS is a highly regarded journal.” Galpin met Hof last year and tried the technique himself. “My whole body immediately felt warm,” Galpin says. “What I think is happening is that you’re increasing oxygen saturation in the muscles and making the body more adaptable to absorb oxygen and perform more effectively.”

Hof prescribes a cold shower, an ice bath, or some other form of cold submersion immediately following his breathing regimen—a peculiar dictate also supported by research. “Cryotherapy increases the hormone norepinephrine,” says biologist Rhonda Patrick. “That ramps up fat metabolism and produces heat as a by-product.” The cold also reduces inflammation and eases chronic pain.

In the past few years, Hof has begun spreading his gospel. , you can sign up for a free tutorial or download his free app to be introduced to the method. If you want further instruction, sign up on the website for ten weeks of video tutorials for $200 or purchase his e-book, Becoming the Iceman, for $15. And while there are hundreds of glowing testimonials online, most researchers say that more studies are needed. “Parts of the reaction patterns in the body are understandable,” says Pierre Capel, an immunologist at Utrecht University in the Netherlands. “But the knowledge about Hof’s methods is not complete.”

Galpin is undeterred. “I’m not going to say it’s a miracle and that after 30 breaths all your problems will go away,” he says. “But I definitely think it’s worth trying.”

For his part, Mackenzie has teamed up with Hamilton to teach surfers, MMA fighters, CrossFitters, and regular athletes how to use Hof’s method. (Hof is an adviser, and Mackenzie and Hamilton still recommend that clients take his video courses.) “The breathing helps with my recovery,” says Mackenzie, “but I also feel better, sleep better, and am overall just happier since trying it.

If You Want to Get a Taste of the Method

Follow these eight steps each morning before food, coffee, or training. To err on the side of caution, do them with a friend nearby.

1. Lie on the ground or sit with your back straight.

2. Inhale deeply, pulling in as much air as you can using your diaphragm.

3. Exhale fully but not forcefully; simply let the breath go.

4. Repeat inhales and exhales for 30 to 40 rounds with your own rhythm.

5. On the last round, exhale and then hold your breath until your body feels the need to breathe.

6. Inhale deeply, then hold your breath for ten seconds.

7. Repeat steps 3–6 for three or four rounds.

8. After your final round, hop in a cold shower. On your first try, stay under the water for 30 seconds, then gradually increase until you reach three to five minutes.