The keystone to an effective swing is to harmonize the rotation of the body and the swinging motion of the arms. Hitting balls with your feet together is a simple and helpful drill that helps you to get the feeling of these two components working together, providing the drill is done properly. The key is to focus on the balance points and make sure you rotate your body and not just swing your club from side to side with your arms and hands.
Just try hitting a few sets of 10 balls with a 7-iron, playing the ball off a tee-peg to get you going. Then go back to hitting balls normally with a regular stance. Try and maintain the balanced feel in your full swing; this should help build consistency and control.
This is something that is very consistent in all good players so the next time you are at the range give it a try, but remember to rotate the body as well as swing the arms.