Powder based face paints are paints that can be usually applied without the use of any water. Think of them as giant eye shadows. To activate powder based face paints simply rub a velvet blender (large eye shadow applicator) back and forward gently over the surface of your cake and then gently rub the applicator against the skin. If needed you can use a primer like lotion so that the powder has an easier time sticking to the skin and so that you get a more opaque and vibrant coverage.
Some powder based face paints like Ben Nye MagiCakes and Mehron StarBlends can be used with water for a bolder look, although once water is used you will need to remove a layer of the cake in order to use them again as pressed powders.
Powder based face paints are great for very soft and smooth blending, creating sheer effects, dry blending and for hot humid summer days since they do not melt or drip on your face like regular water activated face paints might. Powders can also be a great time saving tool since they do not need to dry after you apply a base, so you can go over your base work with the line work right away.
Unless you use powders wet, powders can not be used to do to line work, but you can use your regular water activated face paints or alcohol based face paints to do line work right on top of these powders.