Behind The Curtain

When you take a photo using a flash, coupled with a slow shutter speed, a moving subject will appear sharp. In addition, the movement within the photo, when using the slow shutter speed, appears blurred. This effect is called ghosting. Photographers use this technique to show dynamic, dramatic action within the frame. However, using this method creates some problems.
Typically, the flash synchronization is set to fire at the beginning of the exposure. Therefore, this leads to the ghostly blur being in front of the moving subject. Which in turn makes the ghostly blur of motion appear to be going backwards. To fix this predicament, so the blur will trail the subject, one needs to set the flash synchronization on their camera to trailing or second-curtain sync. This mode allows the flash to fire near the end of the exposure and corrects the problem.
Check your camera's menu options or see if there is an external switch on your camera for this operation. And, as a last resort, you can check your camera's owner's manual. Use shutter speeds from 1/8-1/30 of a second for walkers, runners, and the like.