Some of my Flickr mates posted some motion blur photos, and being a copycat, with no original thoughts of my own, I decided to try it too. You can capture this effect in camera by moving the camera up/down during a longish exposure. I don't mean a time exposure but a shutter speed just long enough to capture the camera movement. You can also accomplish this using the motion blur filter in Photoshop. For me and probably most people, it's easier in Photoshop where one can immediately see the results on a large monitor, and if you don't like them, you simply start over.
In this one ↓ I duplicated the original after blurring it. I then flipped it to create a mirror image.
For the next one ↓ I masked some of the original foreground back in as it would have been almost lost in the blurring.