On an FX camera a non DX lenses focal length is exactly the lens size in millimetres. So, a 300mm AF-S non DX lens will have a focal length of 300mm. However, if you transfer that lens to a DX body e.g. D2x, D300, D7000 etc then the "smaller or APS-C sensor" gives a factor of x1.5, so on a DX camera the lens is 450mm equivilent focal length. If on the D700 or D800 one uses the DX facility then the camera only uses the same area of sensor (approximately) as that of a DX camera. The disadvantage mainly is one ends up with a small file size, approximately 5Mb on DX mode on a D700 because only a proportion of the FX sensor is used.
So, as I have all e.g.Full frame FX lenses then on my D700 they match the focal length of the lens. By extra reach I meant that the lens gets closer to the subject because of it's increased focal length on a DX body.
That's my very simple, not expansive way of explaining it. I am sure others will expand on it.