what is a flash pasteurization?Heating and cooling of the product using equipment designed to do it very quickly. It causes much less flavor change. I've had flash pasteurized and unpasteurized and I think many people could not tell the difference.
when cider is heat pasteurized the sugars (and most of the flavor) is boiled off, and then sugar is addedThis doesn't make any sense, sugars can't be boiled off. Volitile flavors and aromas can be boiled off, but with most equipment the process is enclosed so the vapors are recondensed into the liquid. Also, pasteurization never get the liquid near boiling.