This is a video taken directly from an excellent PDF found at http://www.thelightsrightstudio.com/tutorials-pdf.htm where a couple of techniques using a saturation mask are discussed: pulling back saturation in an image where only one channel was over-exposed (thus clipped or “blown out”) and a second technique to add saturation through the saturation mask to an image already possessing strongly saturated regions that would be over-done if the hue-saturation adjustment layer were applied overall.
This technique obtains its saturation mask from an HSB channel using a filter found on the first of the “Extras” disks that comes with the Adobe CS installation disks (and a similarly working plugin can be found online as well, that works on flattened duplicates only).
HSB stands for Hue (mapped by this filter to the Red channel), Saturation (mapped to the green channel) and Brightness (mapped to the Blue channel, being somewhat like the L channel of the LAB mode). The Saturation/Green channel is extracted from a duplicate of the image to which this filter is applied into the original and is then control (command on a Mac)-clicked to turn it into a selection before adding a Hue-Saturation adjustment layer (which takes on the selection as a mask).
Now, you can pull down the saturation ONLY of the overly saturated regions and restore the detail, shift the reddish clipping back towards green and lighten up the yellows alone.
Additionally, you can use the same saturation mask with a Selective Color adjustment layer to pump the yellows back up a bit to overcome some of the graying effects of the lightness slider in the Hue-Saturation adjustment. When using this kind of saturation mask to control where you ADD saturation to keep from saturating already saturated regions, you apply it INVERTED (by checking the checkbox at the bottom of the dialog when you right-clicked the saturation/green channel to apply it to the original).
Now, having control/command clicked the resulting mask to turn it into a selection, you create a Hue-Saturation adjustment layer (which takes on the selection as a mask) and pump up the saturation to taste, leaving the already saturated regions as they were, but giving much more “pop” and color to the less saturated areas.
The tutorial and many other great PDF tutorials can be found at the above-mentioned website. Enjoy!