Non-Metallic-Metal, or NMM, is a technique that tries to trick the eye into thinking something is metallic when painted with regular matte paints. The highlights and shadows are painted in a way that appears metallic. I absolutely have not figured this technique out yet, so I will once again have to rely on second-hand material.
For thousands of years in traditional art, non-metallic-metal was really the only way to paint metallic surfaces in artwork. Today, we do have paints that contain reflective pigments that simulate actual metal, however the problem - when dealing with models that are only an inch tall - is scale. A full-size, shiny silver sword naturally has very distinct contrast of light and shadow. These details do not show up the same in a 28mm scale model.
To get around this, skilled miniature painters rely on the same techniques used for centuries to trick the eye into seeing something shiny. The same general rules as for any type of highlight/shadow painting apply, with a few caveats: extreme, nearly-white highlights are a must, to simulate the glint of white off of sharp edges. Also, the juxtaposition of your brightest highlights against your darkest shadows is actually required (usually you build these up more gradually), again to simulate the shine and spark of polished metal.