Basically that 50% gray layer can have any effect on it that works with LUMINENCE (dark or light). And, again the benefit: it won't hurt your original pixels! So it is a very useful trick.

If people are interested I am willing to do a ScreenCast tutorial of this - it will be a good excuse to try out my new TUTORIAL CREATION MACHINE.

Yeah yeah yeah. Just do it.
Don't forget to throw in some extras that are important to newbs that don't get discussed much.
Like linking layers, and how to turn a selection into a path.