It comes from a 2004 book by John W. Barrett called Such a Little Secret. It centers around an old BBC broadcast of the Open Championship, with commentary by Peter Alliss and Henry Longhurst, where they discussed "The Secret" as it was being demonstrated by some player who Barrett couldn't remember.
Anyway, according to the Dynamic Duo, "The Secret" is illustrated by this diagram from the book:
To restate "The Secret" so both lefties and righties can use it, "Swing around a still trailing ear." To quote Barrett:
In any focus of concentration, the smaller the focal point the greater is one's sensitivity to it. To continue to use the word ear would do, and in a television broadcast it was probably a more genteel choice than use of the vernacular. Distilling the focal point down to the quarter inch diameter of the aural canal, however, will give a golfer a far sharper image in the mind than the more vague outline of the whole of the right ear. [p57]Look, am I saying that this is "The Secret" to fixing your golf swing? Absolutely not. I don't believe there's one technique that will make you the next Jason Day or Rory McIlroy. If there's a secret, it lies in relaxation, balance and rhythm.
Still, some of you might find some use in this idea of swinging around your trailing ear. You won't reverse pivot or sway during your backswing if you use it, but your head will still have some freedom of movement. You won't drive too hard toward the target on your downswing because, if you do, your head will get pushed away from the target and you're going to fall over backward. And you'll finish in a balanced position -- you need only to look at the drawings above to see that.
So while I don't believe this is "The Secret" to good golf, I do think it's an image that might help some of you who are struggling to stay steady over the ball during your swing. Take it for what it's worth.