#206 Saturation Curve (not slider)
Sometimes, particularly when the luminosity curve is used, mid-tones become over/under saturated (attachment image 1 - oversaturated).
This is presumably why the saturation slider is located with the luminosity curve. However, using the slider affects the entire range of tones, so correcting saturation that way can have undesirable effects (attachment image 2 - stony skin, pale lips, red highlight on hair diminished).
When using a curve to correct saturation, one can reduce the intermediate levels of saturation without having too much effect on the highs and lows (attachment image 3 - vibrant skin, lips and hair-light). It is also possible, as in the example given, to add impact by further saturating the more saturated areas (lips), and reduce the appearance of shiny skin by adding a little saturation to the least-saturated areas (highlights on the cheek, chin, forehead, nose, etc.), all while reducing the overall saturation of the intermediate areas, with a single, simple curve.
I've used GIMP to generate the sample image. To try it for yourself you'll need to duplicate the background layer of a base image. Set the copy's layer mode to "Saturation" and put it on top of an all white layer. Merge these two layers and set the result's layer mode to "Saturation". Ensure this layer is directly on top of the original background, then apply the curves tool to the saturation layer. So long as you keep the upper-right point where it is, this should act like an up-side down saturation curve (pulling it down increases saturation, pushing it up decreases it). I've found this technique valuable for restoring natural/flattering skin-tones in a number of difficult lighting situations, but it's quite cumbersome (and an 8-bit operation in GIMP) , so it would be great if it could be incorporated into UFRaw.
Sample images
And I've just realised those sample images should appear horizontally, not vertically, to minimise the effects of vertical positioning on LCD monitors. Maybe just rotate the entire image on your screen if you're using an LCD.