Gerald: Thank you: so the enigma is really not so arcane: it's only acid that is needed to cause the fixer to become a reducer. (Apparently, there is nothing so special about the 'citric' type?) I will never forget an incident that happened back in the 60s when I was a teen-ager, about 15. I used #2 Kodabromide for paper and was very naive about darkroom matters. I processed the print but it was a tiny bit too dark. My mother called me for lunch and I simply left that print in the full strength fixer (Kodak Fixer powder did not differentiate between film and paper fixer: they were the same strength). After lunch I went down to the basement to my darkroom and was flabbergasted to find a print in the fixer that had the most beautiful tones that I had ever seen. The acid fixer had 'reduced' the density and did so in a 'cutting' fashion that yielded such beautiful tonality and rich contrast. I will never forget that print. - David Lyga
Thiosulfate bleaching action is proportional to acidity, and gives proportional bleaching, not cutting.