It is a way of controlling your Bright, dark and mid tone area's in PS, just looking at it looks like you can also control the darkness of colors which I am very interested in . For eg you can lighten blue or darken just that color, there are a lot of different ways to do it, this is just another way to play in PS
This photo was done the normal way I do it, but in the end I ran a midtone adjustment mask
As you can see it darkened the midtones and left the lighter areas as is. There are other ways of doing it with selective color and darkening, Me I like to play
Here are some links
http://www.naturephotographers.net/arti ... 908-1.html
http://lukezeme.com/tutorials/true-lumi ... oshop-cs6/