The double pendulum model is a way of looking at the golf swing in terms of the various physical elements in rotation, specifically, the two pendulums attached end to end: the arms and hands, and the golf club itself. In the proper golf swing, the arms pivot like a pendulum around the shoulder fulcrum while the pendulum of the club is pivoting around the fulcrum of the wrists. The cumulative motion of the two pendulums magnify each other for maximum speed and force as the club head passes through the ball.
Begin with a stable setup, with a straight line from left shoulder to club head (except for natural wrist hinge in the vertical plane). In other words, when viewed from the front by an onlooker directly opposite the ball from you, your arm and the club should be in line.
Begin your takeaway by swinging equally on both pendulum fulcrums. So, when your club has made 90 degrees of turn and is parallel to the ground, pointing away from the fairway, you should have 45 degrees of turn in your shoulders and 45 degrees in your wrists. Your left arm should be at a 45-degree angle when the club is horizontal.
Continue the process as your back swing continues, so that when the club is rotated 180 degrees and pointing out, up and behind your right shoulder, shoulders and wrists have both turned 90 degrees. Your left arm should be horizontal at this point.
Begin your downswing with a movement of your lower body, which pulls the first pendulum into motion (anchored at your shoulder). As you swing the first pendulum, your locked left arm, back into the original position, allow the second pendulum to lag behind naturally. The angle at the second fulcrum (at your left wrist) between the two pendulums will increase during the first half of the downswing, until your arms are approximately horizontal.
Continue swinging your arms toward the ball and allow the second hinge to begin swinging in its natural motion. The second pendulum (the club shaft and club head) will now begin to swing at a greater rate than the first pendulum, eventually returning to a straight position at the moment of impact. At that time, you should have returned to the original line from left shoulder to left wrist to club head, allowing the club head to accelerate through impact with the force of the momentum of both pendulums.