Luminance Masking - Page 9
Duplicate the Background layer once more and load the "Alpha 1" alpha channel. Press "Ctrl + Shift + I" (Windows) or "Cmd + Shift + I" (Mac) to inverse the selection. Click on the "Make a Layer Mask" icon in the Layer palette. Rename this Layer "Shadows". Change the Blending Mode to "Screen".
After a tweak to the tone of the Layer Mask of the Shadows layer using a curve, I decided that the overall effect was a little too strong. For the Shadows layer, I moved the Opacity to 60% so the effect was more subtle. You may choose to do this for the Highlights layer as well, depending on the particular image.