Interesting...I picked mine up at a model train store, but this might be the same thing. It’s literally a very fine powder. You can apply it with a dry brush. Just scoop some where you’d like it with the brush, then use the brush to rub it into the model where you want to see the effect. Use a tad more than you think you need, a lot falls off in the process (so do it over something like a box top so you can collect and re-use it) but what sticks really stays on well.
You get a really nice mat effect. I use it a lot for 40k vehicles and weapons (black and rust colored powders show nicely on metallic colors) but you could totally do the same with fantasy. Anywhere you want rust or dirt. It’s sort of a ‘softer’ effect than washes (which I also love).