You can use a diamond rod in all sorts of ways. It's just an abrasive. It does what it does. And it's only as good as the user. What works for you is what works.

Using a circular motion is a valid technique when using a large blade on a small stone. You hold the stone steady, choose your angle, and move the blade in small circles, sharpening a portion at a time. When you can feel the blade grab the stone a little, you move to the next section of the blade. This method gives you a lot of control, a more consistent angle, and a pretty good "feel" for the status of the edge.

But if it's just a minor touch-up on an existing edge, you might use it more like a kitchen steel. Especially if you're adept at maintaining a consistent angle freehand (which most people don't do well).