The easiest way to see this effect is to take an image that isn't fully saturated and then apply a strong curve to increase contrast, first with a normal blend and then with a luminosity blend. Unfortunately, from what I've read, the GIMP doesn't have a luminosity blending mode, although someone posted that you can use the "value" parameter. If it has L*a*b, you can do the same thing: apply the curve in the normal workspace and then convert the original to L*a*b and apply the same curve.