These images show the use of fast and slow shutter speed.
The two images that you can clearly see the legs were taken using the fast shutter speed. This is useful as it can take a clear image of something that is moving extremely fast. In my two photos, the subject is either jumping off of a wall or running down the stairs.
My other two images are the of the same thing; running or jumping, however, I had the camera set up on slow shutter speed. This has then caused the image to look more like a ghost as the photo takes a lot slower. They are also have high key lighting as the shutter is closing slower, so more light is being let into the camera.