I don't know for sure, but in some cases he may be using Hue/Saturation adjustments (more saturation) selectively applied using the mask. Example: Add a Hue/Sat adjustment layer, set Saturation to +50, ctrl + I (to hide the effect), airbrush on the mask with white to reveal the effect.
There is another popular technique called "paint with light" you might want to investigate. For a lot of information on this, use the following Google search arguments:
photoshop "paint with light" photoshop pwl
...also check
www.atncentral.com, where you will find some actions to assist with the layer setup.