Tucking a towel under the left armpit and keeping it attached to the body during the golf swing is an advanced practice drill that helps the player keep his arm glued to his body during a swing. Although this motion can be difficult at first, emphasis on fundamentals can make this drill relatively easy to complete.
Place a towel securely under your left arm, tucked close to your body and held tight by the pressure from your left shoulder and armpit.
Address the ball, with an emphasis on fundamentals. The golf swing is a circle with the center of that circle being the middle of the left shoulder. The left arm forms a straight line down the shaft to the golf ball.
Rotate your shoulders away from the target to start the backswing. Do not pull your arms back or reach out over the ball, or the towel will drop.
Make a full turn and coil, keeping your left arm straight and right arm attached to your body. If this position is proper at the top of your swing, the towel should remain attached.
Rotate your hips through the golf ball while driving your hands and arms back down toward the address position. If your left him moves out and up, clearing out space for the arms and club, the towel will remain attached.
Follow through to a full finish, keeping your arms attached to your body. If there is no unnecessary reaching throughout these motions, the towel should remain exactly where it started.
Continue practicing if the towel does indeed drop. It takes many repetitions -- sometimes even weeks and months of practice -- for golfers to learn the feeling of swinging around their body on a flatter swing plane from the inside.