The board is pushing against the ground (and the load!). If you adjust that force depending on the tilt of the board, you can move it (up to a certain point) to stay under the load.

As others have pointed out, it depends a bit on how much lateral force you can achieve through tilting and reaction forces, but if you can balance a quadcopter, you can balance a hoverboard. It gets a lot easier if your load is attached to the platform.