That's a good question. All papers require a little bit of exposure before they will start to record any tones at all. This exposure just takes the paper to the point where tones start to appear. (The effects of flashing are more evident in highlights than shadows.) Where there are very bright highlights the negative is so "dense" that not enough light gets through onto the paper to push it up beyond this threshold level, leaving those areas blank in the print. Pre-flashing lifts the whole sheet (if it's all exposed to the flashing light - you can pre-flash selected areas) up to that level so that all tones record with any subsequent exposure.
Originally Posted by SMBooth
If you don't pre-flash but expose for a longer time then you end up with a print that is too dark. Imagine that you've made a print where all the tones, except for the brightest highlights, look perfect. The brightest highlights were too dense on the negative to reach the threshold level and are a blank white in the developed print. If you give these highlights enough exposure so that you get a tone on the paper, all the other tones that were perfect also get this additional exposure and become too dark.
I suspect you were asking about giving extra exposure by just burning in the very light area rather than exposing the whole sheet? This is possible but there's always the chance that the burning in will be evident in the final print as a "halo" unless done very skillfully. You can mitigate this possibility by burning in at a low contrast grade if you're using variable contrast paper. Not all areas that require burning in are OK with the low-contrast look, however. Burning in at a normal or higher contrast grade is much more obvious and has to be done very well if there's to be no tell-tale halo. In batwister's print above, it would be quite straightforward to burn in at a soft grade. Since distant scenes are often of lower contrast because of atmospheric haze, this would look OK in batwister's print. If you saw Omar's pic, even though a low contrast look would be OK for the blank areas of the print, you can see how much more difficult it would be to burn in the windows. That would require some accurate masking. Definitely easier just to flash the paper.