One of the most common errors that I see when giving lessons is the person who "casts" the club from the top to start the downswing. The shots that result are usually in two categories. First, there is the person who takes a divot that starts about a foot before getting to the ball. The second, which I see more often, is the person who begins to raise their body out of position to compensate for the clubhead traveling too close to the ground, resulting in the topping of the ball.
Why does this swing flaw occur in the first place? In my experience, those who cast the club are often people who desire to hit the ball further. In order to gain distance, somewhere along the line these golfers have been told that they need to increase their clubhead speed. While this definitely can help, knowing how to increase clubhead speed is crucial to this working. Instead, many golfers will try to get the clubhead down to the ball as fast as they can. Ironically, instead of increasing clubhead speed, and improving the power in their swing, they instead lose power on the way to the ball.
By casting the club, the clubhead moves quickly back and away from the target. This will cause one to lose power in the swing, and by the time the clubhead reaches the ball, all the energy has been spent. What results is a ball hit poorly with little power.
How can this be fixed? Well, there needs to be an understanding that the clubhead will always follow whatever the other end of the club does. So, the grip actually should lead or direct the club towards the ball. Once the golf club reaches the "set" position at the top of the backswing (a right angle between the left hand and the shaft of the club) that position should stay the same until the hands and the grip of the club are about even with the golf ball. At that point the hands will begin to unhinge and release down into the golf ball.
By swinging the club this way, one can store up their energy during the swing and then let it all out where it matters the most, at impact! A drill you can try that will help with a "casting" problem is to take an iron and hold it from the wrong end of the club so that the grip end is away from you. For a right handed golfer, stand next to a wall with the wall being to your right. Hold the club in the set position at the top and slowly swing the club so that the grip is moving towards the wall. (remember, you are holding the club upside down) Make sure that when swinging properly and NOT casting the club, there is room for the grip to pass by the wall. Now try to swing at normal speed. When doing this drill, if you cast the club, you will hit the wall. Try moving as close to the wall as possible without hitting the wall.
Note: Try to do this on a wall that will not mark such as brick. I have marked up our indoor hitting bays pretty bad with grip marks on the paint showing others this drill!