The piece of paper tells the app the position of the animation and is removed by the app from the video in the iPad. If you look closely you'll see the app proj

The piece of paper tells the app the position of the animation and is removed by the app from the video in the iPad. If you look closely you'll see the app projects some kind of wooden surface where the piece of paper was, this could be a pattern recognition thing (looking at the surrounding pattern and filling in the paper with that pattern) or just a custom job.