This photo of Trevor Immelman shows that he has maintained great separation between his upper and lower bodies while turning around a stable base. Notice that his belt buckle points just to the right of the ball, while his shoulders have rotated 90_¡ to his toe line. This rotational difference between his hips and shoulders is what Jim McLean called the "X-Factor." The greater that differential is, the farther you'll hit it. Immelman has a large X-Factor, while most amateurs who aren't very flexible have smaller ones. For a young, fit (and smaller) golfer like Immelman, a big X-Factor is vital to hit long drives.
At the top of the swing, he's building power in his core in preparation for his downswing. But look closer and you'll see that Immelman has already made the first move of his downswing. See how he has bumped his left hip slightly toward the target? This move is followed by a hip rotation. After that, his shoulders will follow. Immelman has a very efficient and powerful swing with great sequencing.
Kevin Na generates power from his upper body, in particular, his arms and hands.
Notice that his clubhead has released before his hips have completely cleared. He likely has a strong right hand that helps square the clubface at impact—all characteristics of an arms-and-hands swing. This swing works great for young players with considerable upper-body strength.
With a swing like this, you usually see more forward spine tilt at address and less thigh angle. Also his shoulders probably had very little right tilt, and the butt of his club pointed close to his midline.
His backswing begins with his left shoulder working under and a release of his hips with a bit of lateral motion. The lateral motion toward the target that you see here is a reflection of that backswing.
Na's swing is a young person's swing or that of a well-conditioned older person. The downside to his motion is a higher incidence of shoulder, elbow, wrist and back injuries.
Many Tour players use a "core motion" swing to control their short-iron shots, and in this photo, 46-year-old Paul Goydos demonstrates the result beautifully (although this type of swing works with all shot lengths). Goydos' arms and hands are taken out of the swing, and instead, he releases his body through impact and into the finish.
Notice how short his finish is and how the club remains in front of his chest? He could return to this exact setup position, and his arms and club would be in the same position. This reflects that his body has released, versus the arms, hands and clubhead. This type of swing preserves the body when used through the bag. It requires precise balance at address, but it's easy on the body and minimizes the risk of injury. If you're over 30, I encourage you to use your large muscles in your swing like Goydos. You'll add years to your game.