Where Lootens used the term Flashing, he meant something quite different. He isn't talking about raising the exposure to a threshold to hold a delicate highlight. He wrote about a pictorial effect which he used to darken corners dramatically. He does mention all four times when you can flash paper; before, during or after enlarger exposure and also after print is in the developer - he cautions you about the risk of Solarizing (Sabbatier effect). He recommends flashing after enlarger exposure but doesn't explain.
Originally Posted by Rudolf Karachun
If I had to guess his reason to recommend flashing after exposing in the enlarger... I imagine you can have a clear short-term memory mental image of "where" the exposure has already happened, so a better chance of flashing the correct corners.
I don't use flashing as a general rule. So, without experience, I can't add knowledge of whether "before, during or after" makes any difference. I am certain there is a sensitometrically-measurable difference. But I am guessing there is little difference visible on the print, and that the distinction exists to explain the way of working.