Now when I say center I am talking about what Ki Aikido may call One Point which is situated a bit below the navel, outside the body.
Everything has a center, spin a chair or whatever and you will see it turns around it's center and thus the center is always the calm point around which everything turns.
Are you sure you mean "outside the body"? When I spin a chair it usually rotates around an axis, not a point. So, do you mean center of mass, the intersection of the rotational axes or something else?