How did they do it?

I’m very impressed by the loading technique: images are obviously loaded on the runtime but unlike in the famous resizing slideshow the stayed cached so the site works significantly faster if you let it simmer for a while. But what I’m truly transfixed by, is the wipe in/out transition (I figured out colors fade). Is it the bg covering the image from left and right or the image width and x position expanding and contracting (or both)?
Another thing: no matter how far from each other buttons are positions, images seem to be placed next to each other when called (are they in stack on different levels and then buttons just swap their depths as needed?)