Today I learned a little about the hue and saturation adjustment.
When I scan in RGB and then convert to grayscale I noticed that the shadows usually get so dark that there is no detail. I just learned that, instead of converting to grayscale, lowering the saturation level does the same thing but it does not cause the shadows to turn black. Plus you have a midway ground between full RGB and full gray scale to play around in.
My question is, is lowering the saturation level the elimination of color? Nothing more and nothing less? And why, when the shadows turn black in grayscale, why do they retain detail in a desaturated RGB scale?

Desaturating is just lowering the intensity of the color. Converting to grayscale is desaturating, but there are different algorithms (math) used to reduce the colors. Sometimes when reducing the color, some colors become the same, because they have the same density. I used to have a sweater that my mom had made me, with denim blue yarn and a lightning bolt on it made with vivid red yarn. When a picture of me was in a newspaper, wearing that sweater, it came up as a dark grey sweater with no lightning bolt at all. Some algorithms reduce the density of "some" colors and allow smaller details to stand out from each other.

When you go to Colors > Desaturate in Gimp, you'll see three different algorithms, to allow you to try and keep the details that are important to you. I often use a photo plugin to custom adjust my black and white images.