IME and IMO, freshwater dips do little more than provide temporary relief.
The idea is that the parasites can last only a short period of time in fresh water before they burst, as their osmoregulatory systems are so primitive. The fish will be fine for a longer period, so in theory the fish comes out stressed, but unscathed, and parasite free.
However, an increased slime coat is part of most fishes natural defense mechanisms against parasites of this sort...the dark irony is that this thicker slime coat actually serves to protect many of the parasites from the fresh water. Parasites in the fishes gills are pretty effectively removed using this method, but those on the body live through it.
It can be a good tool to offer relief for a fish that is having a particularly difficult time, but is so very far from a cure.