In the backswing for a right-handed golfer, the right arm or trail arm sets the club at the top of the swing. Ideally your right arm will form 90-angle in your right elbow. As your downswing begins, the right arm drops to your right side, and stays close to your body. The angle in the right elbow may even get more narrow as you maintain the wrist angle.

If your trail arm goes away from your body as you initiate the downswing and you release the angle in your elbow and wrist, you will lose power and the clubface will be out of position.

Hold a club with only your trail arm.
Set up in your golf stance and place your other hand on your hip or behind your back.
Swing the club to the top so that the trail arm forms a 90-degree angle.
Initiate your downswing with the lower body as your trail arm drops closer to your body.
The elbow remains bent until you rotate your forearm to square the clubface at impact.
Follow-through so that the club hits you between your neck and shoulder to finish.
Repeat motion slowly for 8 to 10 repetitions.

This drill can be done with or without hitting a ball. This is an isometric exercise that will not only train the proper movement but will tone muscles in the hips, shoulders and chest.