I have never used it, but I'd imagine that you want to add it in the secondary or before bottling to correct the acidity level if you feel it is needed. That's just my guess though.
I usually use 1 tsp per gallon in apple wine. I add it in primary when I mix up the must. I use it in most fruit wines, unless I'm using a very acidic fruit with lots of citric acid in the wine (like lemons or oranges added to dandelion wine).