Apple Color 1.0 - Manual (Page 194)

194 Chapter 9 Primary In
These two control points roughly correspond to the shadow and highlight contrast
controls. If you add a third control point to the luma curve somewhere in the center,
you can adjust the distribution of midtones that fall between the black and white
points. This adjustment is similar to that using the Midtones contrast control. Moving
this middle control point up raises the distribution of midtones, lightening the image
while leaving the white and black points pinned in place.
Moving the same control point down lowers the distribution of midtones, darkening
the image while leaving the white and black points pinned in place.
While these three control points can mimic the functionality of the Shadow, Midtone,
and HIghlight contrast controls, the true power of curves comes from the ability to add
several control points to make targeted adjustments to the lightness of specific tonal
regions in the image.