I did a really spiffy reply that just damnit disappeared somewhere...

What I read says that cryo extends the heat treating process past room temp into colder regimes, allowing more of the austenite to convert to martensite, and by precipitating eta carbides (very hard substances - diamond is a carbide) into favorable places in the iron crystalline matrix which greatly increases wear resistance. Internal stresses are removed. The piece becomes more dimensionally stable, effectively harder, and tougher.

Not responsible for advice not taken...