Flashing is not a realistic substitute for masking, which can control contrast much more precisely. But flashing can be used for minor contrast tweaks. I don't recommend doing either until the basics of sheet film duping are learned first. To some extent, contrast can be modified by choice of film. Porta 160 is going to be softer than 400. Don't mess with Ektar for this kind of thing. Digital printing is a different path altogether and an obvious option for those who prefer it to darkroom work.