This tool uses "intelligent saturation", so it does more than simply affecting normal saturation values. The positive values (attained when the slider is moved to the right) are comparable to what third-party software often refers to as Vibrance (it is gentler to the skin tones and will be able to enhance, for instance, a blue sky without over-saturating the rest of the image). The negative values represent regular saturation settings. Decreasing the saturation will ultimately turn an image black and white. This, in turn, will change the histogram from RGB to monochrome, although the image will remain in RGB color space as chosen by the output color space.