Don't rely on the barre: Finis Jhung, once a principal with Harkness Ballet, is 75; here, teaching for the gruelling show "Billy Elliot." "Mr. Jhung sets little store by barre work and the attendant grabbing for support. 'Always be in balance, always in control,' he urged his charges — emphasizing the spring action of the bent knee in plié for powerful jumps, turns and combinations. 'I’m not interested in textbook figures,' Mr. Jhung said. 'I’m interested in the mechanics of movement.'"