Wow, that's a cool setup, never saw such a cabinet before. It's the same general principal as a grain separator, the grain and chaff is sent thru a curtain of air. The grain is heavier than the chaff, so it falls into a 'good' bin while the lighter chaff is carried away by the air flow velocity. Good abrasive media is heavier than dust, rust, paint chips/ powder, etc. Someplace in that mechanism, the combined mixture is sent thru an air current where the heavy 'good' abrasive falls out, and the lighter dust, etc is carried into the filter. It's hard to tell in the cartoon diagram where that happens exactly, but it does.