I could ask ten players what starts the downswing, and I would get ten different answers. It has been said that golf is both an art and a science. "To know a thing by its parts is science: to feel it as a whole is art." It helps to understand the mechanics (Science) of a swing. It also helps to understand how to develop the feel (Art).
Once the motion in your swing has started, there should be no interruption. It should be a smooth flowing motion from start to finish; not a series of abrupt actions. To teach your muscles the proper feel is to realize that the physical movements of the body determine how one swings the club. This is why a correct grip is extremely important since it's the only contact you have with the club and controls the clubface angle.
Next lesson: how to achieve the proper grip!