This effect is used to adjust the color levels similar to the 3 Point Balance but with finer control. Although the effect defaults to the RGB channel, it is mostly used with the Luma channel. See also the Curves effect that does the same thing but doesn’t have the curve handles.
Adjusts luminance or color channel intensity with curve level mapping.
Note the extended handles on the red dot in the middle with which you can adjust the steepness of the curve leading to and leaving the point.
See TheDiveO blog for an example of how to use this effect to colour grade clips.