Originally Posted by County Mike
My mind is blown!
I've been watching it for a few minutes and I think the animation is fudged a bit in the diagonal cut. I don't think the chocolate pieces would actually fit back together the way they are cut. I believe they removed the last few frames just before the pieces go back together and your eye is just assuming that it's fitting together properly. Pay attention to the second chocolate "square" from the bottom on the far left and look at how much of it has been chopped off, then look how little of it is being replaced. There is no way that those pieces would fit together to create a full piece, but your mind is being tricked into thinking that's what's happening because you are supposedly seeing it happen right before your eyes.
So, just enough of each diagonally cut rectangle has been removed to essentially add up to an extra piece of chocolate.