Preflashing is a rather specialized contrast reduction technique. Anchell gives a good discussion of it and several useful variations in "The Variable Contrast Printing Manual," and Rudman gives a detailed explanation in "The Photographer's Master Printing Course." It generally comes into play when the shadows are OK but the highlights are too dense to print properly. Contrast reduction is more pronounced in the highlights, noticeable in the midtones, and almost non-existant in the shadows when the technique is applied correctly. This comes about from the additive nature of the non-focused flashing exposure and the image exposure. You have to experiment to find the right level of flash. The idea is pretty well explained above, and generally the point just before the exposure causes any increased density is the right point, but it can vary depending on your aims. Often preflashing is combined with an increase in the contrast grade to get a balanced picture.