higher yield strength as it was originally designed to run the austenite to martensite transformation to a greater degree of completion. Since then, a ton of claims have been made about it doing a whole host of other things to a ton of other materials... they may be ligitimate but I've always gotten a vague snake oil vibe from it all. Just my opinion... in all fairness I haven't read every bit of available literature on the subject.
I've always been weary of the stress relief aspect of it as well... stress is due to the interatomic spacing being greater or less than the lowest energy spacing. To change that you need to rearrange the the atoms to a more stable configuration. Rearrangement requires activation energy which is supplied thermally. (Hence stress relief heat treatments) Cryo treatment reduces mobility which is the opposite of what you want to do. Martensitic transformation doesn't require transport so it responds to the increased stability of this phase at lower temperatures. If the stress state changes after a treatment, I would expect its due the the martensitic transformation which causes a volume change.
