What a wide arc feels like: To feel width in your swing, pull a towel tight with both hands and mimic your swing.
Creating and maintaining good width in your swing will help you hit longer and straighter tee shots. Simply put, a wider swing arc covers more distance than a narrow one and allows you to maximize clubhead speed.
To increase the width in your swing, try this isometric exercise. While standing in your address position, grab a towel at two spots about a foot apart (as shown), keeping a little slack in it. Then stretch the towel as tightly as you can and maintain that stretch as you simulate your backswing and downswing. Notice how your left and right sides keep their distance from each other as they pull in opposing directions. This is the same feeling of width you want to re-create when you swing.
On the range, alternate between practicing the towel exercise and hitting shots until you get the feel of a wider swing arc. During rounds, you can reinforce this feeling using your golf towel while you wait between shots. Once you learn to maximize the width in your swing, you'll start to produce more clubhead speed at impact and rip those tee shots.
*Based at ChampionsGate near Orlando, Leadbetter runs 27 academies worldwide.