Apple Color 1.0 - Manual (Page 191)

Chapter 9 Primary In 191
If part of a curve is raised by one or more control points, then the tonal area of the
image that corresponds to the source values within the curve are being adjusted to a
higher value. In other words, that part of the image is lightened.
If part of a curve is lowered with one or more control points, then the tonal area of the
image that corresponds to the source values within the curve are being adjusted to a
lower value. In other words, that part of the image is darkened.
Editing Control Points and B-Splines
By default, each curve has two control points. The bottom-left control point is the black
point and the top-right control point is the white point for that channel. These two
control points anchor the bottom and top of each curve.
Effect of raising midtones using the Luma curve
Effect of lowering midtones using the Luma curve