[Mr Bill]

But what is actually happening on a chemical level with these marketed replenishers. Is it an alkali to get the PH back up where it should be? Are there compounds that react with redox byproducts and clear them up or sludge them out? Or is it just a fresh jolt of whatever the reducer was?

Essentially the byproducts are diluted out with replenisher. (The excess volume either overflows or is carried over into the following solution.) Compared to the processing tank solution, the replenisher is a bit overconcentrated with both developing agent and preservative. Also the pH is higher to boost the tank pH back up a bit.

In the systems that are replenished with original developer, obviously the initial tank solution must change during processing, and should eventually reach some different equilibrium levels. I'd guess that the B&W systems have a large error tolerance which allows this sort of thing. My experience has mainly been in large scale color neg and paper systems, which are generally too finicky for this sort of thing.