I wonder what's under this thingy?

There is a steel or stainless steel plate on the bottom of the sander which is called the platen. The platen is a flat surface that backs-up the sanding belt and without a firm or semi-firm platen, the belt can't create a flat surface. In this case the platen is backed-up by a cork backer, which is common. Below the cork is a waffle-like casting, which makes sense. Had this been a solid surface it would have been difficult for Makita to grind it flat due to warping caused by the heat of grinding. By making this waffle-like they can easily grind it flat, and the cork will smooth out the surface of the platen. Nice work Makita.