This drill lets you know if you're correctly driving your right knee past the midline of your stance. This is an important downswing move that allows you to turn your torso through impact and get solid contact in the middle of the clubface and a penetrating ball flight. You know you're doing it right if you feel your right knee hit the shaft just before impact. If you don't drive your right knee, it's impossible to turn through the ball. Your thighbone plugs into your pelvis, so if you don't drive it toward the target, your pelvis and core can't turn.
The Move That Makes It Happen
If you have difficulty driving your right knee forward on your downswing, you're stranding weight on your right side and swinging with just your arms and hands. At the top, make sure your weight is on the inside rim of your right foot, and then drive your right thigh toward the target. Setting your weight like this and driving your thigh gives you the leg lean you're looking for, and automatically unlocks the power of your core.