HSV Equalizer
The HSV Equalizer (Hue, Saturation and Value) works in the HSV color space. Use it to selectively change colors, e.g. make the sky more dramatic by making it somewhat darker or more saturated without touching other colors. It can be very useful for correcting skin tones as well. 'Incorrect' use may produce effects resembling the analogue cross-processing technique. When used for black-and-white, be sure that the raw is desaturated by setting the Chromaticity slider in the Lab Adjustments tool to -100, or by enabling the Black-and-White tool. The HSV Equalizer does not work when the raw file is desaturated by setting Saturation in the Exposure tool to -100.
There is a description of how the flat curve type works in the General Comments About Some Toolbox Widgets chapter.