I should probably say keep your elbows in, but that title isn’t as catchy. When stunting as a base, cradles excluded, elbows should stay inside of the shoulders. Power is lost when the elbows get wider than the shoulders. I see this being a problem most often on twisting up to group stunts, such as full ups, and the toss of coed style stunts. Focusing on keeping the elbows as close to the center of the body as possible during stunts will keep the top person’s body better aligned, making the stunts easier to hit.
I’ve heard several coaches emphasize the need to keep elbows in, but probably none more than Tony Crump, current coach at the University of Memphis, and Saleem Habash, former coach of the University of Kentucky and Dunbar High School.