From a NYCB dancer in the early, early days:
Balanchine “had a way of demanding, of asking you to do just a little bit more than you thought you could, of keeping you on edge, keeping that carrot in front of you. And that’s what the audience felt. When these dancers got on stage, they were dancing to the edge of their ability. They were going over the precipice.”
I’m transcribing the entire interview for publication but it’s going to take a while.