They are images being pushed into a canvas element and then they are stepping through them as you fake scroll. Fake scroll, meaning they take over scrolling duties from the browser. Other elements such as the text are layered on top of the canvas and then are parallaxed into/out of view as you scroll.
It's quite a simple and elegant way to do it, other than the incredibly large number of images required to pull it off. You are essentially watching a video frame-by-frame.
As for the smoothness of it, your mileage may vary based on various things to do with your personal setup.
For me it is not smooth on Firefox. It's choppy but still presentable. The experience is much better in Chrome on my Mac.
It's quite a simple and elegant way to do it, other than the incredibly large number of images required to pull it off. You are essentially watching a video frame-by-frame.
As for the smoothness of it, your mileage may vary based on various things to do with your personal setup.
For me it is not smooth on Firefox. It's choppy but still presentable. The experience is much better in Chrome on my Mac.