In addition to reducing density, potassium ferricyanide is also used in formulas for sepia and blue toning—which can be a bit jarring at first, since the raw salt is a very bright red color.

Potassium ferricyanide is normally safe to work with, but contact with weak acid (e.g. the stop bath used in film and print development) will release hydrogen cyanide gas, which can kill very quickly.

Most striking to me, to bring out highlights in the sky, potassium ferricyanide was needed to physically reduce density on the negative, whereas nowadays we can simply use a low-contrast filter for the highlights and a high-contrast filter for the shadows.

For more info see:
https://www.mlb.com/news/cheerleading-has-become-part-of-taiwan-baseball-culture