On her way to the Twin Cities, Cynthia Neilson's toe rings set off airport security alarms.

"Arms up!" the attendant said, as she checked Neilson for contraband.

Instinctively, Neilson's arms shot up, as if she were doing a yoga pose. The attendant started laughing. She recognized the posture; she practiced yoga, too.

This weekend, Neilson and nearly 600 of the country's most flexible bodies and calmest minds are in St. Paul for the Iyengar Yoga Convention, a first for Minnesota. Teachers and students will breathe, stretch and ruminate on the philosophy of their practice. Named after its 85-year-old creator, B.K.S. Iyengar, this form of yoga is known for its precision, alignment and life lessons, practitioners say.

"Iyengar is like the Harvard school of yoga," said Neilson, a yoga teacher from Dana Point, Calif., who's been teaching 30 years, six in Iyengar. "Once you get the foundation, that's when the freedom comes."

On Thursday, Iyengar instructors took classes to sharpen their teaching skills and move forward in the certification process. The Iyengar system has 13 levels of certification, from teacher-in-training to advanced senior levels. The higher levels require a minimum of nine years of teaching and several study trips to India, where Iyengar lives.

"It's really rigorous," says Momi Jhung, an instructor from Minnetonka.

Twenty people — 17 women and three men — gathered Thursday for Chris Saudek's teacher-in-training class. Saudek, from La Crescent, Minn., is one of the most senior teachers in Minnesota. Statewide, 10 people are certified up to the intermediate senior level, according to convention co-chair Kristin Chirhart.

Purple, blue, green and orange sticky mats were crowded side by side, but no one seemed to mind. Their blankets, straps, blocks and other props lined a far wall. They had an ambitious agenda of 12 postures, with poses building on previous ones. Unlike yoga classes in health clubs, there is no music. The goal is to quiet the mind.

After a morning invocation, Saudek starts with virasana, or hero, pose. For the uninitiated, this is a sitting posture.

"Put the block between your feet.

"Put the knees together.

"Hands in the crease.

"Pull the calves out.

"Sit down.

"Spread your toes well.

"Hands on your legs.

"Sit up.

"Roll shoulders back.

"Draw your shoulder blades into your back.

"This is virasana."

For the next two hours, Saudek demonstrates, explains, demonstrates again, critiques and monitors the postures. Little escapes her. When she spies a woman standing with her arms crossed, she chastises her: That posture closes your chest and heart, and will send the wrong message to her students back home.

Nadine Sims, a bookkeeper and yoga instructor from Bend, Ore., says people often think yoga is mainly a physical discipline, but it's more.

"Beginners think they're training their body, but they're really training the mind," Sims says.

For example, doing a backbend can help people overcome fear, a skill they can use in other areas of their life.

Hugh Ross of Shelby Township, Mich., has been practicing yoga for 25 years and teaching Iyengar since 1986. Ross drives 124 miles roundtrip to take classes in Ann Arbor, Mich., the closest city with certified Iyengar teachers. Ross says yoga has provided him with energy, serenity and other benefits.

"I can fit into the same clothes as in high school," he says.

IF YOU GO

What: 2004 Iyengar Yoga Convention

When: Saturday to Tuesday

Where: RiverCentre in St. Paul

Cost: $475