Bow Down to the Yoga TeacherBruce Katz / Amelia Island Plantation-MCT-Getty ImagesGreg Gumucio, standing, leads a yoga class at his studio, Yoga to the People, in New York.

Marco, the tattooed instructor at the front of the room, is all charisma. He stalks; he pounces; he perches on my back as he corrects my [...]