I did a series of photos with this effect for the WHS yearbook a few years ago. There are two special techniques being used. One is called “dragging the shutter.” What happens is that if you turn your flash on but manually adjust the shutter speed to a fairly slow speed, you can sometimes get some interest ghosting in the image. You can see that in the green t-shirt in the center of the photo. The other technique is to zoom in on some central subject while you push the exposure button. This creates a natural radial blur in the photo. I took 8 or 10 pictures before I got the effect I wanted.
Canon 5D 1/15s f/3.2 ISO100 75mm