From Jack Keller's web site:
"Invert Sugar: The product of the hydrolysis of sucrose, which is glucose and fructose. Dextrose (an isomer of glucose) and levulose (an isomer of fructose) are obtained by the inversion of sucrose, and hence called invert sugar. Yeast convert invert sugar more rapidly than sucrose, such as simple cane sugar, because they do not have to break the sucrose down into glucose and fructose themselves. Invert sugar can be made by dissolving two parts sugar into one part water, adding two teaspoons lemon juice per pound of sugar, bringing this almost to a boil, and holding it there for 30 minutes (NOT allowing it to boil). If not to be used immediately upon cooling, this can be poured into a sealable jar, sealed and cooled in the refrigerator. Invert sugar should NOT be used to sweeten finished wine as it will encourage refermentation."
I usually make invert sugar (simple syrup) to adjust SG at the beginning of fermentation for the reason Mr. Keller states. It is easier for the yeast to convert it to alcohol. I have used both simple syrup and granulated sugar to back sweeten with the same result, clear wine with no residual sugar. I think it's exactly what Dave said, just stir the daylights out of it. With Skeeter Pee, I always use granulated sugar. With other wines, it just depends on whether or not I have any simple syrup already made up and stored in the fridge. The stuff will keep almost indefinitely in the fridge, a gallon jug just takes up a lot of space.
Greg