In the "open-to-closed" method that Jack employed and was used by Vardon, Snead, Nelson, Jones, and most other great players, the club is taken away in one piece and the clubface gradually opens going back and closes on the downswing. Jack felt that this was the easiest method to learn, the easiest on you physically, and the method that would allow you to play well the longest.
The other method is what Jack called the "closed-to-open" method where the clubface is held closed, or facing the ball on the way back and held open on the way through. It was employed by Lee Trevino and Arnold Palmer in his early days. Bobby Jones called it the "shut-faced" method that was best employed by Leo Diegel in his day. The "closed-to-open" method, according to Jack, required greater strength.
I have tried both methods and have found that I have difficulty hitting a fade with the "shut-faced" method. I also tend to hit the ball lower and predominantly with a draw with the closed-to-open method. I am either no longer strong enough, or quick enough with my legs to hit my favoured fade with the shut-faced swing.
Jack preferred his method, but admitted that he sometimes used the shut-faced method, presumably when he was playing a hook or a draw. Both methods have been, and still are, used successfully. But Jack believes that history has favoured his method. A good modern example of the open-to-closed method might be Henrik Stenson. A top echelon shut-faced player would be Zach Johnson.
In the end, as Jack was sure to point out, the only thing that mattered was getting impact right. He just felt his way was easier for most people to employ. I hope that makes it a little more clear. I guess what would work better is a video. Sometimes words just don't effectively get the point across. However, I have included the drawing on the subject from Jack's book, Golf My Way.