It's either actually, based on what is more applicable:
When the flares go off they create shadowing. As they descend, the shadows move. The higher the round goes off, they less dramatic the movement. This gives you some wiggle room based off of whether you are surrounded by conceilment (say in some bushes) or standing in an open field (I would reccomend dropping down in that case).
Incidentally, you are supposed to close your shooting eye to maintain night vision. Which goes to the flamer issue; short flashes of light do not wash out your night vision for long. Some, such as a strobe, don't affect it at all because it is too quick. In either case, because your eyes have adaped to the dark they recover quickly.