Well on the forehand (if we are talking about the modern forehand) the elbow plays a key role in getting the racquet to the contact point and through the ball.
If you watch Roddick sideways you will see his elbow driving the rotation. In the past, or an intermediate forehand, the elbow moves with the shoulders - it is more "fixed" together. Kind of like Jimmy Conners forehand.
When you drop the elbow in place and are ready to bring the racquet forward, the shoulder knudges or "pulls" the butt cap forward first. This starts the elbow lead. It is a very slight lead because it would be awkward to keep it going forward without turning your back shoulder into the contact zone.
When the elbow is knudged the shoulders lag. This lag helps reduce the chance of overrotating past the contact point. The leading of the elbow helps to move the elbow in front of the body plane when you make contact.
Here is an example of what I mean:
Several things to notice:
1. The leg action is supporting the upper body uncoiling into the ball. Tommy has a wide base in which to rotate over and keep his balance so he doesnt "tip over" if his feet where too close together. Remember on rotation there is a lot of centrifugal force going on that forces things out of the contact zone, so having a wide base helps control this. This is true for a twohanders rotation and a onehanders "non-rotation.
2. Watch the elbow how it loops and drops for the forward swing. It will be "knudged" or pulled forward first, then the shoulders almost immediately follows. The elbow goes in front of the body plane at contact. That is the goal! The shoulders cant overrotate if you do this right. But like anything else - it take practice. First start out hitting slow balls to get the feel of the stroke down. After a month or two hit medium paced balls and see if your stroke breaksdown. If it does slow down the balls and repeat.
3. The knee bend and lift move like a spring not a jump or hop! Tommys legs begin lifting once they are bent slightly before the up swing to the ball, but they dont "pop" up to throw off the racquet path.
4. The fixed wrist is maintained through contact and then releases after the ball has been hit. If you watched this is regular speed your eyes would only pick up the after effects of the shot and it would look real whippy. Because the pros can generate a ton of racquet head speed this becomes an illusion to the human eye.
5. Watch his head. It barely moves once it is locked into position. Now for the forehand, the head is a little more open, but it stays still allowing the eyes to pick up the ball properly.
6. The use of the non-dominant arm is essential. It provides balance and helps with tracking and gauging depth and the speed of the incoming ball. You get more of your body into the stroke.
I hope that helps, if not keeping explaining what you mean.