When we watch good golfers swing a golf club, it's easy to be tricked into seeing something that isn't occurring. Golf is full of optical illusions. One GREAT example that is a key fundamental is what the arms do on the backswing and downswing. Speaking for myself, I've struggled with pulling the club WAY TOO FAR behind me on the backswing, because I concluded this was the proper motion by looking at still images of great professionals in their backswings. I felt--because of what I thought I observed--that the arms were made to move around the body horizontally--in a circle (a problem which can cause one to be laid off). And then I would do all sorts of weird contortions on the downswing to correct for the awful backswing (usually with disastrous results).
Now, let me say that, yes, there are many different kinds of backswings and downswings among great players, but there's only one kind of GOOD impact. And many pros do things that they correct through some compensation (e.g., look at Jim Furyk's backswing and follow-on downswing as an example). But all of these guys and gals end up with great contact. Hey, they're professional athletes. And I'm not!
At any rate, I feel a top fundamental for us mortal golfers is to keep the arms in front of the body at all times--takeaway, backswing, transition, downswing, and followthrough (and that's using all clubs and situations). This means the arms, wrists, and club simply lift the club up and lower it down vertically in front of the body, while the club is moved horizontally by the backswing pivot and downswing. Shawn Clement and Steven Bahn's videos explains it best, though you can find many other such examples on youtube (including this one) and elsewhere.
Drill (Proper Motion): Take your address position with a 7-iron and then stand up from the hips; the club should be roughly parallel to the ground. Now, without any body pivot at all, cock the wrists upward; the left wrist with a neutral grip will be slightly cupped. Now bend the right elbow slightly (<=90 deg) while keeping pressure in PP1 (right palm on left thumb) to ensure the left arm is straight. You'll notice the arms and hands raise almost vertically in front of the torso only a few inches. Practice this up-and-down motion for a few moments. This represents the correct vertical motion of the arms, wrists, and club during the golf swing--somewhat like hammering or chopping wood. Coupled and timed with the correct torso turn back-and-through, this is the entire swing--a simple motion.
Drill (Improper Motion): Perform the same address procedure in the previous drill, standing erect as before. Now raise the arms while moving the hands to the right, pulling the left arm tightly across the chest. The right arm bends more. The left arm will tend to collapse. The hands move to the outside of the right shoulder. This is more of a horizontal motion and is what causes the club to get laid off and stuck.
The arms DO NOT need to provide the horizontal or circular movement, because the pivot does that; the arms, wrists, and club should move roughly up-and-down in front of the chest during the entire swing and move around simply because of the pivot. If you catch yourself pushing OUT on the shaft with your right arm to get back to the ball, you're trying to supply horizontal motion with your arms and you'll be OTT.
Your arms and hands don't move very much during a full swing.
Think about it. The hands are in the center of your body (ie., in front of your belt buckle) at setup. When the hands are waist-high, they're still in the center of your body. That means your shoulders have moved and not much else. Then, from the waist-high position, your right elbow bends and your left elbow doesn't, which causes you left shoulder joint to pivot up.
So your hands have only moved from your waist to just above your right shoulder, and most of that movement comes from bending your right elbow. That's not much movement at all!
...while maintaining your address posture, lift the club and lay it on your right shoulder next to your neck, with your elbows bent and close together (2). Now turn your shoulders and hips away from the target exactly as you would if you were making a full backswing (3). When your turn is complete, simply extend your left arm out until it's nearly straight (4).
Clark is so accurate, even under pressure, partly because he swings with his elbows exceptionally close together. He had a physical problem as a child that forced him to keep his elbows together, but those close elbows have become a blessing to his swing. Most amateurs have elbows flying around in the downswing, and that causes your club to go off plane and your shots to go wayward. By keeping your elbows close together, especially in the downswing, you'll find it's much easier to have a repetitive motion in which you keep your wrists cocked and the club on plane.
THE CLUB STAYS IN FRONT OF ME
My coach, Chuck Cook, says I do a great job of keeping the club in front of my body throughout the swing. Amateurs tend to swing mostly with their arms, so the body lags behind; better players can get fast with the body turn, so the arms lag behind. If you focus on turning back and through with the club staying in front of your chest, you'll find it easier to square the clubface and hit straighter shots.