There are a number of techniques for isolating and manipulating just the shadow-- or under exposed-- areas of your image. This one is a favorite, because it you have complete control over the area and the ability to work with any number of tools.
I like the low-key quality of the image above, but her black hair disappears against the black drop. I could have added more rim light, but that wouldn't have teased out the texture very well. Or I could have increased the front light, but then I would have lost the moodiness of the low-key. If I had really wanted to be finicky during the shoot, I could have added a front hair light, and lowered the more general light on her face to compensate. If you remember your light theory, you'll recall that light is additive. But this wasn't an advertising shoot-- I didn't have half an hour to fiddle with one shot. So what can I do in post?
First-- as always-- duplicate your layer.
Second-- you need to select just the shadow areas. If you press Ctrl + alt + 2 on your keyboard, you'll select just the bright areas. On older versions of Photoshop I believe it's Ctrl + alt + ~ but they changed this shortcut more than once. If neither of those work for you, google that command with your version of Photoshop. This will select the highlights as shown below.
The great thing about this command is that the brighter the pixel, the more it's selected. In other words, it's a gradation.
Third-- reverse (or inverse) the selection: