First, let me start by saying there is no right or wrong when it comes to a golf swing, there is simply cause and effect. If the clubface is open, square or closed to the target line at the moment of impact the golf ball will start in that direction and the golf ball is in contact with the club face at impact for 4,000ths of a second. These are laws of physics, how we take the club away from the ball and return it is not. The one and two-piece golf swings are an interpretation of how I see better players swinging a golf club and how this may help to identify what works best for you.
The one-piece golf swing is where the body and arms reverse direction and start the downswing before the club has reached its apex, or top of the swing. There is no pausing or stopping in this swing form. Examples of this would be most of the earlier golf swings when shafts were more flexible and the hands were used much more. Names such as Jones and Hogan come to mind, and more recently Garcia, Duffner and Fowler. An advantage of the one-piece golf swing is it definitely produces lag, a very important piece of the power puzzle. However, timing is a bit more difficult to repeat and disconnection is often a by-product. Downloading is created when the body and arms start the downswing before the club reaches the top, which in turn creates lag where the wrists can, and often do, over hinge. This over hinging is called disconnection, where the club disconnects from the rest of the body. When this action is mastered, however, as many great players over time have done it can create great speed for a player who is shorter in stature or lacks great strength.
The two-piece golf swing is basically a backswing and a forward swing separated by a slight pause, or technically speaking a pendulum motion. This is a more modern swing action with examples such as Love, Couples, Els, Choi and Speith. The beauty of this swing is the simplicity and ease of squaring the clubface at impact as well as the consistency of repetition. This 1-2 motion is by far the easiest way to swing a golf club. Power, or speed, in this form is created by a big shoulder turn and good foot action.