The third act of Swan Lake is considered one of the most difficult in the balletic cannon, requiring a dancer to twirl around 32 consecutive times. How its accomplished is as much a feat of physics as it is athleticism.
Staying in momentum for a full 30 seconds takes a couple of small but important considerations on the part of the dancer. First and foremost is keeping a consistent center of gravity and straight turning axis—otherwise all that momentum will throw her off balance on onto the floor.
The real trick comes from some very precise limb actions: an occasional push with the point foot, how long the rotating leg stays fully extended, and where her arms are in relation to the center of her body. Using all three in a carefully choreographed series of movements, the dancer can maintain just enough angular momentum to keep spinning but in a way that looks fluid to an audience. And theoretically, she could keep going like that forever, aside from some small mitigating factors like nausea and exhaustion.