Is there a description of the "Show Luminance Map" option in the Masking environment?

I think it's presenting the same luminance map you'd see with the Alt Amount slider in Sharpening.. 

Help says: 

Select the Show Luminance Mask check box to view the luminance information of the photo in black and white representation. The part in red color shows the actual area masked by the Luminance Range Mask.

So Richard is right with no Hue, but also no Saturation.

I think this is intended to suspend hue information from the image, such that the selectivity of the masking within the image, vs the image itself, can be better seen. Otherwise things can get very visually muddled in practice - assuming both mask and image are even both previewed together, which may not be the case depending on current Overlay setting.