You can use the accelerometer to figure out which way is "down". Then, using CV techniques to identify the ground plane. Even if you get this a bit wrong, as lo

You can use the accelerometer to figure out which way is "down". Then, using CV techniques to identify the ground plane. Even if you get this a bit wrong, as long as you visually track some registration points, you get consistent positioning of your virtual object, which is probably the most important bit. I noticed in the videos that nothing ever goes in front of the animated objects, so they're probably not identifying objects, and trying to figure out object boundaries or anything like that. It's a simple, well-polished trick.