What happens of you don't? The sensors tend to interpret artifacts in the sensor's data (like digital noise) as motion, and send instructions to the sensor to compensate. This produces an effect sometimes called ghost motion. The result is that the image stabilization system introduces motion blur into a photo that would otherwise have been as sharp as a tack.

So the bottom line is that you should turn image stabilization off when shooting with a tripod, but leave it on the rest of the time. You can tell when image stabilization is doing its job when you depress the shutter release halfway; the viewfinder image should tend to "freeze" as the system dampens the normal jitters of your body.

You might be able to exert even more control over your image-stabilized lens besides just turning it on or off. Some lenses have Active and Normal modes, like on my Nikon 18-200mm zoom.

Active is a good all-around setting for counteracting ordinary jiggles both vertically and horizontally. If you're trying to , though, you'd want to switch to Normal mode, which compensates for vertical vibration while allowing for the kind of side-to-side motion you are trying to capture. Various lenses are marked differently, so check your user guide for details about the lingo on your particular lens.