Thinking about the wheels makes sense now...

Simple physics tells you that when you throw back weight you are propelled forwards. Now make a 'light' frame and add a heavy weight that swings down and backwards. The frame will, if on practical wheels, move forwards...
Now I didn't think of this as I didn't believe the wheels would be rolling well enough for this, or the weight to be propelled far enough backwards to send the frame forwards. But obviously I was wrong.