The book of Pyro by Gordon Hutchings has been my film Pyro development bible since it was released and I have been using Jobo and pyro for 15 plus years and can easily state that we have processed thousands of runs of pyro .

But here is my layman take on what happens.. Ralph/ Ian, take over please if I am not explaining the chemical and physical details correctly.
Pyro is a tannin developer, and my understanding is as the silver is developing out the bloom effect is lessoned and the silver migrates less due to a hardening of the gelatin around the grain, which kind of stops migration the effect gives much more defined highlight detail.
***we must remember and I think this gets lost on a lot of people is that within highlight regions we are not only talking about white, but grey and black objects that are within this region.***
As development goes on these silver tones get hardened and set very nicely within the main highlight region as well. This adds tremendous local contrast which can be seen as greator detail in the highlight region. This has been pointed out many times by Les Mclean.

for example go into a room lit only by a light bulb and photograph the room.
with a normal developer the bulb will not be visible and their will be all kinds of flare around the bulb.
with a pyro developer the bulb will be visible and very little flare around the bulb.
There are those who argue that graded papers are the best and I do remember when all the papers were graded, a scene of extreme tonal range would be impossible not to sacrifice at one end , and make a much lower contrast print.
If you tried to keep super rich blacks with good separation and burn in the highlights you would get mushy , soft highlights.
This is why I split print on VC paper and use a Pyro Developer for photographers working with extreme or long tonal ranges in the original scene.

Graeme
If your friend is using the same developer as you and it is not Pyro as you state then I am not sure why the difference. Agitation can have a tremendous effect in the highlight region which would cause softening in the highlights, otherwise I do not have an answer.

My clients these days have been with me for years, together we have discussed the options available for their particular needs and try to match the ISo and developer for certain applications.

Flat scenes we never use Pyro, but rather D76 or HC110 .

Medium scenes we make the decision based on shutter speed required.

Extreme scenes we usually go to expose for shadow , open one stop , and drop process for the highlights.
I always use a two bath Pyro Developer- 1000 ml + 1000 ml using the exhausted first developer as the stain.
We have not strayed from these principles . I do have a hankering to try the Pyrocat Semi Stand, as I have seen some awesome negatives made this way but with client work I cannot bring myself to experiment with their film.
Sandy King and I believe Steve Sherman use this method of film development for their work and the negs are good.

Quote Originally Posted by GraemeMitchell View Post
Bob, could you elaborate on what this means when using non-staining devs?

I mean, say w/ d-76 or HC-110, are you saying while developing that the silver and image structure migrates in the highlights...? Do you think this have any correlation to the old practice of keeping the wet time of the film to a minimum (something that's been argued as false here, but which I was always taught..."don't let the grain swim around!")?

I ask, b/c there's a fellow I know and respect who runs very very very good film. He and I use the same dev, and when on rare occasion I've had him run my film, the one major difference I've noticed is how much sharper, acute in grain, and more tonal separation, he achieves in the highlights and say zone 7 and up.

I've often wondered what the difference is in how we handle the film...and I thought he may just develop to a higher CI more aggressively, but your above statement about silver migrating in the highlights made me wonder if there could be other things at play.

Apologies for derailing the thread!