The change over from backswing to downswing is referred to as the transition and in my experience is one of the more difficult areas of the swing to understand. In this case the understanding is not just a cognitive understanding of the positions, but the feel of a good transition and how it works in rhythm and sequence. I would recommend anyone trying this to focus on the five stages of the sequence and how they work together in movement.
Simply the swinging motion in the backswing from a feet together position. This should be done relaxed and not too regimented.
Next as your arms and club are reaching the top part of the backswing the weight shifts to the forward side by stepping your front foot towards the target.
The body rotates smoothly.
The arms and hands swing past and release through the ball (this can be done with or without a ball).
I think this is an important phase so keep the going to a balanced follow through.
Understanding the order of these motions can take a bit of getting used to at first but with some practice it should give you a greater feel of the body wanting to turn toward the target with the swinging motion of the arms as well as understanding good weight transference. This drill is particularly good for those golfers who slice the ball of golfers who have a tendency to finish on the back-foot.
Paul Thompson, Fellow PGA Professional, Powerscourt Golf Club