The flying elbows of Sampras and Lendl are seen in the backswing. Their elbows are as high or higher than their racquets during takeback and are sticking out from the body by quite a bit. As Limpin noted, during the forward swing they snap the elbows down and generate a large rotation about their upper arm, thereby whipping their lower arm (and racquet) into and through the ball.
It's quite a bit different than a modern forehand, and is more designed to hit through the ball than to generate loads of topspin. They both can and did pronate their wrists at contact however, so that part of their stroke has some modern elements, but they both did not allow the racquet to sweep all the way through the WW motion. Both of these guys are using Eastern grips too (I believe).
I understand what Cheetah is talking about leading with the elbow. The elbow does in fact lead the backswing in a modern forehand. Perhaps it's not the mental image that Limipin likes and perhaps you can say it's more the result of other things like a good unit turn with both hands on the racquet. I don't personally think about my elbow leading my backswing though I know it does.