Austentite is the crystaline structure of steel when it is soft. When heat treating, you raise the temp. of the steel above a certain limit, the crystaline structure cahnges to martensite. Martensite is hardened steel. Once you reach the point where the steel is martensite, you quench the steel so as to "freeze" or "lock" the steel into its martensite form.
The sub quench is made by placing the metal in liquid nitrogen for varying lengths of time. Ed Fowler was obviously a bit dubious about it, but proved to himself and Rick Dunkerly clearly that it indeed has considerable benefit to the steel. I just can't remember exactly what that value is. I believe it either changes the martensite level, or makes the grain finer. Something of the sort. I've started re-reading the book, hopefully will remember when I come to that section to study it a bit closer.
I have rad that cryo-quenching steel will help all of the austentite transform to martensite, and you pick up some hardness.
I'd liek to know what Cold Steel means by "Sub-Zero quench", exactly. I hate all of these mystery terms! I like cold steel knives in general, but I don't like it when they change standardized names for marketing purposes.