When you support any item like you do when trying to balance it, if the item's CG is below the support points, the item is positively stable. The CG will try to tilt the item with the CG moving toward the supports. It's stabilizing much like adding noseweight stabilizes the pith axis of an airplane in flight.

When you support an item with the CG above the support points, the item is negatively stable. The CG tries to tilt farther away from the supports.