There are several texts including "Way Beyond Monochrome" that discuss flashing of the paper. Its primarily used for graded papers wherein you want to print a high contrast neg on a lower grade of paper (or in between grade). The higher the grade of paper, the lower the density range of the negative.
I've used the technique on both enlarging paper and contact printing paper. With enlarging paper, you would stop down the lens and give the paper without negative a brief exposure, eg 3 sec (and then place paper in covered container). Then place the negative back in tray, focus and then expose as normal.
For contact printing as with AZO or Lodima, its best to have a timer such as Beseler audible that can give shorter than a second exposure time. Or use the enlarger as above to increase the paper sensitivity.