Pretend the fire dancer is the center of the clock. The cameraman who took the picture we see, was sitting at 3 o’clock. But there was another person with a camera, sitting at 6 o’clock. The 3 o’clock cameraman was relying solely on exposure to capture pictures, while 6 o’clock was relying on flash. Both took the picture at the EXACT same time (or close, since 3 o’clock probably had a tiny bit longer exposure). What you’re seeing is the shadow cast in the lit area from 6 o’clock’s flash, but since it isn’t a magical 360 degree flash, the illuminated space has an end; the left ending is seen right in the middle of the photo. So there is no light from the flash behind the dancer, from 3 o’clock’s point of view.
What’s funny, is that is actually a little tiny bit true.