The rubber band effect is a way to describe the golf swing by understanding it in terms of storing energy by changing shape and then releasing energy in a sudden burst by snapping back to original position. Just as a rubber band can be stretched out and then snapped back into place, a golfer can stretch his body out in the back swing before snapping back to original position as he comes through the ball.
Stretch vs. Coil
The analogy is not a formal one: the motion of the golf swing is primarily a rotary motion, but this does not correspond to twisting a rubber band. Rather, the rubber band effect focuses on the idea of building up energy by changing position into a stretched or extended position, just as a rubber band does when it is stretched.
Potential and Kinetic Energy
The rubber band effect is based on an idea of simple physics: potential versus kinetic energy. Kinetic energy is the energy of motion, the energy any object has when it is moving in one direction or another; potential energy is the energy of condition or position. A boulder at the top of a hill, for example, has potential energy. That potential energy can be converted to kinetic energy if the boulder is pushed off the hill and rolls down. A stretched rubber band also has potential energy, energy that is transformed into kinetic energy when the rubber band is allowed to snap back. In a similar way, a golfer stretches his body, storing potential energy, during the back swing, and then "snaps back into place" in the downswing, converting the potential energy to kinetic energy that can then be transferred to the golf ball.
The key to utilizing the rubber band effect is to attain maximum coil at the moment when most potential energy is stored. That moment is the very top of your back swing, when your club is poised at or near parallel to the ground over your right shoulder. At that moment, your position is analogous to the position of a rubber band stretched to its maximum. All that remains is to allow the mechanism to "snap back" into place, so that as much of the potential energy is converted to kinetic energy at the moment of impact at the bottom of the down swing.
The Plyometric Golf Swing
For the purposes of the rubber band analogy, the golf swing must be understood as a plyometric exercise. A plyometric exercise is a two-part exercise in which the muscles or muscle groups to be exercised are first stretched and then flexed fully. For the golf swing, those muscles are the muscles that control rotation, especially the glutes, abs and back muscles. These muscles are all stretched, perhaps almost to capacity, in the back swing and then flexed to bring the coiled body back to original position. The way to transfer all the potential energy stored at the top of the back swing is to flex all the stretched muscles completely on the downswing, creating maximum club-head speed.
Anatomically, the stretches of the golf swing are limited by the turn of the back. When the back is turned with maximum differential between shoulder turn and hip turn (about 70 degrees for the most flexible golfers), you have reached the trigger point. The rubber band, so to speak, is fully stretched. Any more stretching and you risk snapping the rubber band; that is, greatly destabilizing your swing. At the maximum coil point, begin the drive through the ball, returning to setup position, where the club will drive through the ball.