Bath A is the developer itself, but it isn't activated until you put it in Bath B.
So, the emulsion soaks up developer in Bath A, and when the activator is added in Bath B, the developing action starts.
There is a finite amount of how much developer can get soaked up in Bath A, meaning Bath B will only develop the film so far until there the developer has been depleted. After that point there is no reason to leave it in Bath B any longer.

My experience with Diafine is that it gives a similar tonality every time, but you can't really control anything other than your exposure. With normal single bath developers exposure AND developing time/temp/agitation are both variables that you use to control the final negative contrast and tonality, and one of those controls are now omitted. So, it works really well for some scenarios, and not so well in others. Depending on the lighting you shoot in, or how you want your prints to look, you may or may not like the results.