Before he walked into his first yoga class at the San Quentin Rehabilitation Center, Darnell “Moe” Washington imagined the instructor would look like Richard Simmons—the short, boisterous television fitness guru who wore tight red-white-and-blue athletic shorts.
Instead, he found James Fox, a tall, thoughtful man who commanded the room with his calm demeanor. Mr. Washington, who was serving a life sentence for murder, knew nothing about yoga but figured he could match Mr. Fox pose…