What ? Tales of the tail

I got this email today. So I called my two dogs that were lying in the room with me. Their tails went back and forth. How can a dog wag a tail to one side? I don't get it.
DogAge tip of the Week
A Telling Tail September 27, 2007
The direction of your dog's tail wags may tell you what's going on at the other end, in his brain. Researchers found that dogs consistently wag their tails to the right at the sight of something pleasing and familiar -- their owners, for example. A dog's left brain, like a human one, deals with positive emotions. And because the left brain controls the right side of the body, happy excitement will send a pup's tail wagging to the right. Tails take a left turn when dogs greet someone less familiar or when they encounter intimidating behavior in other dogs. Wagging to the left reflects feelings like fear and anxiety.

So how does a dog wag its tail in only one direction? Is that still called wagging? Unless they mean which way it goes first, which is hard to figure, when it is going so fast. LOL!