Change the surface to a variable, shifty one. A half-inch stack of yoga mats, for example, or a spongy pillow (not too soft). Do it again.
It's supposed to strengthen the stability of the balls of your feet by developing the foot's muscles, ligaments and tendons. It's also supposed to help flat-footed people overcome a serious problem which looms as they age: the problem of losing their balance and spraining, straining or breaking something.
Enjoy. Ow, my feet hurt.