Dan, there are several classes of silver photographic bleaches, with different actions on the silver in the film. (**) For a reversal process, a bleach is needed that converts the exposed silver to a form that is soluble in the bleach. For potassium dichromate + sulfuric acid, the silver is oxidized to silver sulfate.
So in the reversal processing sequence, we have:
1) Unexposed film, no image -- unexposed silver halogen salts.
2) Exposed film, latent image -- exposed and unexposed silver salts.
3) Developed film, silver positive image -- metallic silver and unexposed silver salts.
In normal processing, we'd fix now, removing the unexposed silver salts, leaving behind only a metallic silver image. Reversal, however, continues on like so:
4) Bleached film, metallic silver removed -- unexposed silver salts.
5) Washing and clearing
6) Reexposure, latent image -- exposed silver salts.
7) Redevelopment, silver negative image -- metallic silver.
8) Washing
9) (optional) Hardening
Potassium ferricyanide is used as a re-halogenating bleach, where the silver is converted back to a halogen form (e.g. with bromide to achieve Ag -> AgBr), which must then be removed by fixing. This creates a problem during steps 6 and 7 above, since the silver salts from the bleaching would result in nasty image fog.
(**) I'm going from memory on the bleach info, hopefully someone will correct if I've misstated anything. Mean library
D) took back Mason's
Photographic Processing Chemistry, and my copy of Haist's
Modern Photographic Processing hasn't arrived yet.