He shot that pic at 78mm (Fullframe), which is quite a distance so you get noticable background blur as well. But you still have to move with the subject and use a relative slow shutter to get the proper effect of movement. The horizontal streaks are what you want, they show the direction the camera was moving in (following the rider) Even then its still quite hard to get the photo right as you need to stay focused on one spot of the rider during the whole exposure. Other than that, it is as you described it. Slow shutter and move with the subject. If you put your camera on constant focus it will adjust continiously untill you actually shoot the pic. See original here with all the camera settings http://www.pinkbike.com/photo/5572637/