I call this the Swiss Army Knife motion. Think of it like opening a swiss army knife that has three parts unfold. Open the first and the second and third remain collapsed. Then uncollapse the second one until it is fully opened, leaving the third.

First Roddick drives his elbow up to the ball, while his elbow remains collapsed so that the forearm and racket trail behind. Then he straightens his elbow, opening up the second part of the knife. Do you see the final part that needs to open? It's the racket itself, which has been pulled up to the ball butt first.