Some of you are not answering the question...
There is a difference between a swing singing off stage when they are not on AT ALL, and Mary Testa, who WAS on, singing off stage. Obviously her participation was a creative decision...and if she was out, her understudy would do the same because it's a requirement of her track, not because she's off stage.
And the answer has already been given...most times swings watch or rehearse.
How to properly use its/it's:
Its is the possessive. It's is the contraction for it is...