This ensures that the temperature will not rise high enough to allow the enzymes to activate. Although it cannot hurt you, namazake gone south becomes cloudy, and the flavor can turn cloyingly sweet, acidic and yeast-ridden. This is a good 300 years before Louis Pasteur produced his findings on the subject in France. Namazake is always labeled as such. Somewhere on the bottle will be the easily-recognizable character for nama, and a note that it must be kept refrigerated.
Most sake is pasteurized twice, once just after brewing and once again before shipping.
It retains some of the qualities of flavor and fragrance of fully unpasteurized sake, but has less danger of spoiling. In the end, it is not much more than a marketing ploy, and if you want to try namazake, go with completely unpasteurized namazake. Namazake is easily available now at any good sake shop or department store. Naturally some brands are more often seen than others. Also note that whether or not a sake is pasteurized is unrelated to the grade of the sake.