Worked for me on FireFox/Ubuntu, once I clicked 'enable' on my Flash Blocker. Nice and interactive - moving in/out/right/left/up/down with the mouse.
It looks to be adjusting size and placement and reflections/shadows of each sphere based on an assumed eye distance from the visual plane. And adjusting the focus to match an expected depth-of-field.
There are cartoons on TV that look very 3-D on regular TV, no glasses. I never read what they are doing, but I think it is basically adjusting the relative size of things as they move to give the impression of depth. When something moves towards you, it gets bigger, but when done by computer (I assume) it is all so 'right' that it tricks the brain somewhat. Maybe someone else can share more about it, that's a guess on my part, and is probably only a partial explanation (lighting, shadows?).