Flashing is just adding an overall layer of weak light to the film, like fogging it. It looks like what you'd think it looks like.
When used in very low amounts on the negative, the loss of true black is not too noticeable because of the gamma of the print stock, etc. but it can lift low-level detail into greater visibility. The basic idea is that some information is on the threshold of visibility and just needs a bit more strength (or density on the negative) to cross that threshold. It is related to latensification, the notion that adding some weak overall light gives some barely-exposed silver halide grains enough photons to become developable into silver. Though flashing is more about lifting the blacks; more than a very weak amount (like a 7% flash) and you've gone beyond increasing shadow detail into merely milking the blacks and giving the impression of lower contrast. Flashing also softens color, plus reduces sharpness a bit because it reduces contrast, and contrast and sharpness are interconnected.
There are films that were flashed mildly for a flashed look, and then films that were flashed but then printed using a silver retention process to restore the black, thus canceling the flashed look. The reason in this case for flashing was to counteract the increase in contrast that silver retention printing causes.
"McCabe and Mrs. Miller" was flashed, as was "The Long Goodbye". "The Little Princess" was flashed about 10%.
Then there are the movies that were flashed through the lens using a Lightflex or Varicon, such as some movies shot by Freddie Francis -- "Dune" for example. Often the flash was colored, imparting a tint to the shadows. "Dune" for example used different colors in the Lightflex for the different planet, mostly a brown flash for the desert planet scenes.
The thing is that it's hard to show on a still frame because it's mainly about the black level, and I can't tell if the video transfer messed around with the black levels to get rid or, or reduce, the flash effect, and I can't know what the black levels are like on your computer monitor.
If you look at the Blu-Ray frames here:
The image has mostly been corrected for normal blacks, but in a few frames probably some of the details in the black or dark green costumes are more visible due to the Lightflex flashing.
This review of "The Long Goodbye" has some frames that show the flashed look of the movie:
The DVD contains a reprint of the 1973 American Cinematographer article on the movie - it's a great article because it has strips of the movie print reprinted labelled by the amount of flashing used for each example.