Two of the biggest factors that contribute to distance are hitting the golf ball in the center of the club face, and creating speed. Think of your swing as having two engines - your body and your arms. Both are power producers and both have their role in the swing. The body needs to make a centered turn around your spine with limited up and down or side to side motion, thus creating an efficient coiling of the upper body and lower body. The arms are your second engine and they will swing around your body with speed, but only if they are relaxed. Imagine swinging a string with a rock attached at the end. As your arms swing around in a circle, the rock follows the path of the circle, and continues to increase its speed as your arm increases the speed of the string. In its passive state, the string is limp but when in motion, it stretches out and straightens. For those players who try to keep their arms straight at address or during the swing you are simply losing speed, creating tension, and producing a weaker impact.
Imagine a baseball player's set-up when waiting for their pitch to be delivered (See Image A). Their arms are soft and relaxed so they can generate more bat speed. That is the feeling you need with your setup in golf. At address, the arms should be relaxed and hanging down from your shoulders, tension free. At impact in baseball, the player's arms are stretched out to full extension but it is the swinging motion of the bat that creates this. (As the body turns, the arms swing around the body on an arc as in the golf swing.) At impact, the relaxed arms are stretched out by centrifugal force due to the swinging motion of the arms. The left arm and club reach their straightest point just past impact. This is a result of the arms being relaxed and tension free in the swing. Top Drill Try this drill to generate speed. Take a shaft with some tubing on the end or flip one of your woods upside down and swing the club trying to hear the swoosh at the bottom of the swing. (see Image B) This is the speed you need to help add distance to all of your shots. Compare that sound to the sound of your swing when your arms are stiff, and you will better understand how you can generate swing speed to add distance.