My understanding of it is that it helps steel the most but in other metals it can cause some stress relief and increased uniformity in the metal. Better heat conductivity and less likely to fail. Might make your piston less likely to shatter, especially if its cast, but don't know what it would do for a forged piston thats already so unlikely to break. Bearings, transmission, and crank pin would seem like the most logical things to me to cryo treat.