Is Rachel24601 British, perhaps? Never heard of "corpsing" in the U.S. And "a rolling cart used to move scenery" is a "wagon". Never heard "jackknife" before. A the proscenium is fixed. So if anything can "reduce the opening between the stage and the wings" it is "legs". And "rostrum" is not really used in theatre here. It refers to a dais. In American stagecraft a raised platform is simply called a platform.