Bleach rehalogenates developed silver back into silver halide which you cannot see (for the most part). This is then redevelopable with any developer. On the same token it is also a candidate for removal by fixing. If you bleach a print or film and put it back into developer it'll come right back. One can do this countless times. However if you bleach a print or film and put it into fixer, then the fixer does it's job and removes all undeveloped (which rehalogenated silver is) silver from the print or film. To rehalogenate means to re-halide hence silver->silver halide. All film and paper is silver halide based. If you use a dilute bleach you can rehalogenate only light density silver while leaving the rest relatively unaffected. CLS is part of this light density mask of silver.
I'll let someone else comment on the citric acid part.
That was helpful, thanks, that also made a lot of sense and I'm STARTING to get "this whole film thing" but I'll have to re-read this in the morning.