Use a slower shutter speed than normal and move the camera so the skier is always in the center, because the shutter speed is slow and you're moving the camera, the background will be blurred and the skier will be in focus, it's called Panning like what Kamikaze said in the title.
thats part of it, but what happened in that shot is he used a flash that was very short compared to the overall shutter speed time so the objects close to the camera/flash stuck out, but the images farther away that the flash did not affect as much are more blurred from the panning of the camera.
