Many weekend players struggle to create lag during their backswing and maintain it into the impact zone of the downswing. In the book "How to Feel a Real Golf Swing," Bob Toski writes, "Feeling the club is the key to feeling your swing." Toski is talking primarily about lag. Creating and maintaining lag is a fairly natural action. The problem is that most players don't understand why lag happens or what they should feel.
When your hands reach the top of your backswing, your trailing elbow bends and causes your wrists to cock. The head of your club tries to keep traveling over your shoulder and behind your back, even though you've changed direction and started your downswing. The momentum of the club head works against your effort to make your downswing, and the combination causes your wrists to stay cocked for a while during the downswing. That delayed uncocking of your wrists is called "lag."
Later Wrist Cock
Think about how you swing a hammer or a flyswatter. You bend your elbow to lift the tool, then cock your wrist at the end of your "backswing." In the same way, the clubhead needs to create momentum late in the swing to create lag, so your wrists need to cock later in the swing. You don't need to keep your wrists straight until the last possible moment to create lag, but you don't want to cock them too early in your backswing either. A good backswing will feel as if your wrists are fairly straight until your hands are about waist high in your backswing. That's when your trailing elbow should start to bend, and that's when your wrists will start to cock.
At the Top
When you reach the top of your backswing and change direction, you'll feel the pressure of the club head's momentum in your wrists and particularly in your thumb joints. This pressure helps you find the best tempo for your swing. If you don't feel much pressure, you're probably swinging back a bit too slow. But if you feel too much stress -- or even pain -- in your wrists at the top, your backswing is probably too fast.
Too many players become obsessed with trying to "hold the angle" of their wrists, but the momentum of the club head does that. The key to maintaining lag is in your trailing elbow. Just as when you swing a hammer or flyswatter, your trailing elbow's angle should stay the same or get smaller as you start down. You shouldn't straighten your elbow before it gets near your side (around waist high in your downswing) or you'll lose all your lag. That's called "casting" and many players do it at the very top of their downswing. Your elbow should feel as if it just drops to your side as you start down. Once your elbow is close to your side, you'll have kept enough lag that you can straighten your elbow and hit the ball a long way.