I do this to pop highlights on prints. Doing it on your film will, indeed, slightly pop your shadows, though subtly, and you have to be very attentive to get it right. If you are going to do this, prepare a very weak solution of Farmer's Reducer (at least 1/10 normal working strength), and do not presoak your film. Go straight to the reducer. If you dunk in a dry piece of film, the thinnest areas of the negs will bleach back faster than the thicker areas at first. Keep a very close eye on your film, and have a running stream of water going. Pull your film out as soon as you think the reducer has started to work on the highlights. Rinse it, then fix again. You only have one shot at it, then you need to dry your film and do it again if you still want to cut the shadows more than the highlights. If you lose a little more highlight density than you would like, use a strong selenium bath to pop it back. You can probably add at least one grade of contrast by combining these methods. I do it with prints all the time. I print down slightly dark, and slightly flat. With a dry print, I then bleach, fix, wash, dry several times until the highlights look good. When dry, if the shadows have lost some punch, I will selenium tone.
It is time intensive and tedious, and may not even be the best way to get what you want. But you should at least try it on a crummy shot to see what happens.