Topping the list of "absolutes" in Skagit casting is probably the preservation of tension in the line while it is constantly being pulled around during the sweep and transition before release.Since skagit casting is such an abbreviated move and trying to stay in the box. How important is body and hip rotation,especially in the sweep. Is it necessary?
Any hip and body rotation during these movements must preserve and hopefully enhance tension in the line. Using the hips and body to assist in direction change is fine (and often necessary), just avoid loss of line tension while doing so.
Another poster mentioned using the body as a twisted energy storage device--that is exactly how Ben Hogan described his backswing when teaching golf. The large muscles can store and smoothly release a lot of energy while allowing the hands/arms to remain "quiet", thus avoiding or at least dampening any jerky motions in the smaller body parts.
Whether its your legs, back, hips, torso, shoulders or arms, the "feel" should be that the line is being constantly pulled up until the point of release. If that feeling is lost even for an instant, efficiency is being degraded. In casting, a small movement under great tension is far better than a large movement without tension!