Apple Color 1.5 - Manual (Page 236)

Curve 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.
Curves in Color are edited using B-Splines, which use control points that aren't actually
attached to the curve control to "pull" the curve into different shapes, like a strong magnet
pulling thin wire. For example, here's a curve with a single control point that's raising the
highlights disproportionately to the midtones:
The control point hovering above the curve is pulling the entire curve upward, while the
ends of the curve are pinned in place.
The complexity of a curve is defined by how many control points are exerting influence
on the curve. If two control points are added to either side and moved down, the curve
can be modified as seen below.
236 Chapter 9 The Primary In Room