Weight transfer is simply the movement of the body’s weight from the back foot to the front foot during the golf swing. Virtually all athletic motions are based on the legs using the feet to move the body. Tennis players will serve with their weight loaded on the back foot and then transfer their weight onto their front foot for extra power in the serve. Similarly a boxer who throws a punch will generate more power using a full body rotation with his right side than if he just used his arm to throw the punch, without using his body.
If we relate this to the golf swing, a good backswing is where the golfer turns and shifts his weight away from the target building up a spring like effect between the upper body and a firm lower body. The downswing is driven by the right foot, knee and hip turn. As the spring unwinds, the right hip and leg drive powerfully towards the target followed by the arms which are pulled down on the correct inside path. At impact nearly all the body weight has been driven forwards, towards the target, with 90% of the weight on the left (front) foot. At the finish of a good golf swing with the correct weight transfer the shoulders, chest and hips all face the target with the right foot in a vertical position turned away from the target.
Swing on a Slope drill: To feel the weight shift happening naturally, try swinging while standing on a downhill slope. During the downswing gravity pushes your weight toward the front foot, making it easier to finish the swing with the weight fully transferred. When doing this, be sure to keep your shoulder line parallel with the slope of the ground to avoid hitting behind the ball.
Ball forward drill: Take your normal setup position with the ball placed in the middle of your stance opposite your sternum. Now leaving your club in the same position move the ball forward by 10 cm. Now go ahead and make a swing and see how easy it will be to transfer your weight onto your left foot.