My understanding of flashing for film and paper is that each crystal of silver salt needs something like 3 photons to make it developable. The shadows on a negative or the highlights on a print just don't get enough to develop. They get maybe 1 or 2 photons and won't develop. Hence the toe in a film density curve. Controlled flashing will supply the necessary photons to allow those almost exposed crystals to respond to the developer. The already sensitised crystals of the rest of the image are not affected because of the low light of the flash. The technique works well with contrasty films or scenes and to enable the paper of a print to produce highlight detail that would not be available without it.
Pat Gainer described a setup where he had a light set off to the side of the easel to supply those extra photons. He had determined the parameters by experiment.