Strobist Info: One small pop-up flash from a Nikon DSLR in front. So technically this is off camera flash.
I think it is interesting to see that the whole picture is defined by the intended motion blur as a result of panning. But there is also the flash of another shooter in front. I captured it as I was dragging the shutter. The flash froze part of the biker (look at the camera-left side of his upper arms and his glasses) and a huge patch of street directly in front of the wheel on camera left.