I only use hyposalinity for the treatment of "Ich" and not as a prophylactic. Not that I see anything wrong with hyposalinity treatment for all new fish, but just that I have not found in necessary.
I quarantine new fish for at least 6 weeks. In most cases there are in the quarantine tank much longer simply because it is convienient. Six weeks is long enough that if the new fish is carrying trophonts, they will have dropped off, reproduced and reinfected the fish at least once in the period, making the infection obvious. If "Ich" does appear during the quarantine period, it is easy to treat with hyposalinity.
Here is some detail on
Hyposalinity. As Oama states, you should add the fish to the tank with normal salinity then lower the salinity over the course of a few days.