I wrote this
AH III skins tutorial in the beta forum explaining the new system. Its already slightly out of date though. I said you can submit either a bump map or normal map but actually you have to use a normal map.
Essentially though white on an environmental map will give you a chrome-like effect where the skin reflects the environment around it perfectly, black won't reflect anything. White on the power map gives a very sharp reflection, black a very diffuse one. I find these maps end up looking much like the specular map, but are generally darker. This is because shiny surfaces tend to have both sharper reflections and also reflect the environment more than matt ones.
In AH II power and environmental effects were controlled with text files, in AH III it is done on a per pixel basis rather than globally.