Believe it or not PE is correct when he says Sabattier effect and it is different from Solarisation.
I heard that solarisation took about 1,000 times extra exposure over normal, to produce. I decided to test that statement, it's correct according to me. I used Panatomic-X and exposed it almost directly to the sun. I forget the times but Solarisation happened, The sun turned into a positive on the neg and the building was negative.
Back to the question at hand. I have successfully done Sabattier prints, using Ilford MGIV RC simply in the darkroom.
My standard set-up is a desk lamp with a 15W safelight globe inserted sitting approximately 1.2 metres above the developing tray. This was actuated with an enlarging timer to have precise repeatable results.
First standardise a time for print development, I used 1'30" @ 20C. Check out when the image first appears fully, from that time you can flash the paper with white light, not before. My own white light flashing, is usually between 1/2 a second to about 2 seconds. Depending on where in the developing cycle you flash the paper, is what usually makes or breaks the final outcome. So watch the clock and note when you flash!
You then continue development until 1'30" is up, pull the print smartly, then drop it into a fresh stop bath.
Fix, review the results after the paper is dry.
In a nutshell, that's it.
There are variations, different special developers, myriads of methods, but that is for you to work out.
I also tried this special developer from the Australian company Vanbar
vanbar.com.au
Photochem PC R77 Sabattier Kit pdr
Sells for $9.60 USD (export price)
I wouldn't say it was bad, in fact I got good results from it, but I was and am happy with normal developer and MGIV RC, it's what I have, is cheap and doesn't use much water, (we are in a severe drought)
Mick.