Hypo is when you reduce the level of salanity to under 14 ppt (and yes, you need a refractormeter). I keep my QT tank at 12 ppt. A lot of saltwater dieases, especially ich-cant survive at that low of a level. However it does take 4-6 weeks to break the cycle and for it to die off. It is always best to QT any new fish arrivals for at least 3 weeks, however I am a big fan of 5-6 weeks. Just remember tho that if you run your QT hypo'd-then it is just for fish, as inverts and corals cant survive in that low salanity either.
Make sure you still acclimate your fish slowly using the drip method so they get use to the lower levels.
Hope this helped explain, and good luck.