The Arc Touch Mouse appears to have two modes: curved for ergonomic gaming, and flat for... well, there's definitely got to be a reason it can flatten. Fingers crossed it's a trackpad of some sort. From Gizmodo
That one has been out for some time, not the same one. One Phil posted has a flexable bottom one you linked is a plastic bottom with a hinge to fold in half.
maybe flat for transportation purpose? kinda like laptop. open for use and closed for transporting it