The way I see it, Clive, is that if the amount of exposure thru the negative is x and the amount of flash is y, then x+y and y+x should yield the same amount of exposure. I do not think the silver cares when it is actually exposed first -- thru the neg or flashed without the neg.
The idea of flashing is that the paper has a threshhold of z: the amount of exposure needed to get any tonal change beyond pure white. Flashing is used when x<z in the highlights where one wants some tone. In those areas you want x+y, or y+x, to be greater or equal to z.
In the shadows, the exposure is on the order of 200x or greater (compared to x in the highlights). So 200x+y is not significantly different than 200x.
Fun with x,y, and z!