Apple Color 1.5 - Manual (Page 238)

You can draw a general correspondence between the controls described in Contrast
Adjustment Explained and the black point, midtones, and white point of the Luma curve.
For example, moving the black point of the curve up raises the black point.
Moving the white point of the curve down lowers the white point of the image.
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 of using the Midtone 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.
238 Chapter 9 The Primary In Room