As per the thread title,
What is a contrast mask? I've heard abit about it here and there, but I've never been able to find out what it actually is and what it does.
Can anyone enlighten me please?
If it is a workflow, then it is just a way to make a selection, like regular mask, except you separate based on contrast.
Briefly, what i do is to go the channel palette and check the R, G or B layers which have the best contrast that will help me make the selection (for e.g: selecting a human from a busy background). Doodle a bit to increase the contrast (channel mixer adjustment layer -> monochrome and pump the values of whichever RGB layers has the best contrast, play with blending modes etc), made amendments (painting in certain areas to help with the contrast) and there you have it - a mask!
This is an extremely brief account, but the bottomline is, you want to increase the contrast between your object and the background you want to separate so you can easily create a mask.
Most Channel books should have examples on this. Hope it helps.
On the computer, there's also much more flexibility. E.g. you can use such image-derived masks masks for blending different layers into one picture, allowing you complete control (curves etc.) for each contributing layer which gives much better control than traditional masks.