A few days later, in the dark of morning, I met Heidi at Eddie Stern's Broome Street studio, a peaceful place with no sign on the door, no health-club-type gift shop or thumping aerobics-class music.

Students filter in slowly — on average, 75 to 100 a day — and begin bending and twisting through a precise and fluid regimen of 30 or so postures. Being new, I was allowed only to watch. The room smelled like sweet incense and sweat, and the only sound was the Darth Vader-like rustle of people doing the ujjayi breath.

Something about the whole secretive, exclusive ritual reminded me of what it was like to cruise Azzedine Alaïa's atelier when he first started selling his sexy creations out of a small apartment on the Rue de Bellechasse in Paris. New clients were allowed to shop only if they were brought by friends of the designer or possessed enough nerve to ring the fourth buzzer and brave rejection.

Mr. Stern teaches what is called the Mysore style, where the students move at their own pace under his eye or the watchful eye of his colleague, Russell Kai, who is known not only as the guy in Christy Turlington's book, but also as a strict and thorough teacher. He patiently explained that people can go through a lot of pain and discomfort in the practice, but that students develop a level of fearlessness. "So far as I know, nobody's been crippled yet," he said.

Two days later, Heidi and I were sitting across the street having a cup of jasmine tea at a hangout where Ashtanga students often go to discuss their classes.

"It freaks a lot of people out because it's just you and your practice," Heidi said. "There are no mirrors, and there's no one telling you what to do. It's just you facing yourself. You and your fear."

Kate Betts: Yoga, Unlike Fashion, Is Deep. Right? New York Times (Registrierung erforderlich).