To clarify, there are a couple different meanings of the word "flashing"
Dave referred you to Lootens, who talks about pictorially darkening the edges of the print by holding a card over the print and hitting it with a lot of light (example, 5 seconds of white light from a 7 watt bulb) as you move the card like burning. You do this without the negative, so the effect darkens distracting backgrounds. In one of Lootens examples, the straight print clearly shows the support structure of the backdrop. After flashing the subject is silhouetted with a dark clean background.
The other definition of "flashing" is fogging exposure which brings the whole sheet of paper up to the threshold of turning gray but still remains white. This is a dim or short exposure to light over the whole sheet, and it helps print detail in highlights that might otherwise be blocked. This is the kind of flashing Doremus Scudder was talking about in reference to the Les McClean site.
