Plexi, Diafine is a two-stage developer that has extended compensation characteristics.
Part A is developing agent only, no activator. The only thing that happens here is that developer soaks into the film emulsion. So time and temperature are unimportant, within reason. 3-5 minutes is sufficient.
Part B is the activator only, no developer. The only developer present is in/on the film, so there's a natural limit to the extent of the developing action. Obviously, film areas more heavily exposed will proceed with development action quickly, and then when the developer present in those spots is exhausted, action slows and stops without blocking up the highlights.
In the less-exposed areas of the film, the developer is not soon exhausted, so continues working. This is where the shadow densities improve and true film speed is raised. In this compensating action, the developer works until either exhausted at last or there's no further exposed silver halide upon which to work.
Again, within reason time and temperature are not important. Again, 3-5 minutes is fine. This is pretty handy!
If there is any "trick" to using Diafine, it's in keeping agitation to a minimum, especially in Part B. Excess agitation, I believe, will simply wash the developer away from the highlight areas where it's most needed, leading to thin negs. I just jiggle the tank periodically so that development byproducts that would inhibit activity don't collect adjacent to those areas.
I hope this doesn't come across as confusing. I just find it useful to have a feel for what is going on in the tank!
Diafine is almost designed for Tri-X, as the two work beautifully together. But it works excellently with other films too. Since there is no way to pull or push development though time and temperature, the only adjustment is in choosing an exposure index for the film.
Diafine's instruction sheet has a chart showing appropriate EI for a list of films. Just recently I developed a group of films together in the same tanks, separating them only on the basis of what reels will fit nicely together in the tanks. Verichrome Pan 110 size at EI 80, 35mm Tri-X at EI 1200, Ilford Pan-F at EI 64, Ilford FP4+ at EI 250, and a 120-size roll of Ilford Delta 3200 at EI 1600. The 3200 roll looks a bit thin, all the rest look great.
Diafine is interesting and useful stuff!