He's used a tilt-shift lens, which, very basically, enables the photographer to move the lens itself up/down/left/right (like this: www.luminous-landscape.com/images-77/24-pc.jpg ) while it's still attached to the camera. This changes the plane of focus, often in a 'slanted' way, just like in this picture.
To put that into perspective for this photo, instead of simply focusing on the rider like a normal lens could, this lens enables the photographer, using special knobs on the lens itself, to blur the sky and far right, while still gradually introducing focus into the bottom left of the picture.
Tilt-shift lenses are also used in architecture and product photography, as they can alter the perspective that a person would see from the same height.
Hope this helped! Give it a bit of research if you want to know more, they're amazing (but super-expensive).
(Sources: I'm a photographer.)

 Thanks Domenic, I have heard of tilt shift lenses, but i never got round to figuring out what they do. Im a photographer as well, but im still figuring things out with the more sophisticated functions. Anyways, thanks for the long and informational answer. Ill look into them and possibly try one out sooner or later. Cheers!