What you describe is the Sabattier effect. You flash a light on the print in the developer. This is usually done at about 3/4ths of the way through development and is about 1 - 5 seconds with a white light at about 10W at 5 feet. I use that strange measure because I just take the filter off my safelight and flash the paper.

It generally gives poorer results with MG papers due to the color of the light source which can alter contrast. It is more stable with graded paper.