They're talking about adding compounds to keep the muzzle flash low, usually potassium nitrate or potassium sulfate. Nitrocellulose doesn't have enough oxy in the mix to completely burn the fuel part, so when the hot gases hit the oxy in the air we have the muzzle flash as they finish burning. The additives either add oxy to the mix to promote complete burning within the bbl, or tolower the burn temp and produce a diffused smoke that helps obscure the flash. IIRC, Federal started using a low flash powder in their 125gr .357 loads back in the 80s as the flash in that load was impressive and many LE agencies were still using it. Since then, the practice has extended to almost all loads marketed for self defense. IIRC, Ed Sanow had an article on the .357 load in Firepower magazine back in the day.