It's a couple of things,,,,, The "swing" in a cast is kind of an understudy for the chorus, meaning that the have to know small parts, but MULTIPLE small parts--- very hectic, and challenging. Kudos to all the "swings." A tough job that not many can do. There a also "swings" on the technical crew-- people that rotate between set changing and general behind the scenes work. They fill-in where ever there is a need.