In other words, sit on a cushion and it pushes back against the force of your weight. This sounds counterintuitive, of course, but their design makes some kind of sense. One would expect that any material that behaves this way would be, by definition, unstable--and therefore would collapse into a stable state on its own, thus nullifying whatever characteristics the unstable state possessed. But the Northwestern team designed around this by creating an internal structure that does indeed fall into a stable state. The stable state is just more compressed or expanded than the original state.