There is also a thread elsewhere on this.

It completes martensitic transformation after heat treatment quench but before temper of highly alloyed steels (crank shaft for example). Martensite is the iron-carbon phase that makes heat treated steel hard. Cryogenics (sub cooling in industrial circles) is most effective in very high alloy steel and completely ineffective in plain or mild steel. It also must be incorporated into the heat treatment process to work.

A properly engineered cryogenic process can enhance the fatigue life of a highly stressed component by a substantial factor. A "lets try it and see what happens" approach will most likely result in no change. The fact that it's popular with hobbyists is that they don't understand the cryogenic process, the mechanisms of metal fatigue, and it seems high-tech.

That's the short story.