works because the metal is a mixture i.e an alloy and becasue it has a crystaline structure of atoms that vibrate around a point. and their vibration is dependent on the temperaure of the metal. a smaller vibration of each atom allows it to fit through spaces it couldn't when vibrateing wildly.
if it was pure iron all the iron atoms align into neat rows like a grain over resonable distance, and sit there buzzing away at a level dependent on heat energy, occasionally you come across a boundary where the grain changes direction.
Any in neat rows can move slightly, like popping marbles around each other in the palm of your hand. Which is why cast stuff kinda moves, over a period of years after casting and first machining. The whole concept of a seasoned block is dependent on this idea
If you hammer, machine or heat the block you promote movement. It changes shape slightly. and the more movement there is, the more likley atoms move to positions where they get stuck, the boundaries between the grain move and tangle up, and the block esentially gets stiffer.
Basically you increase the instances of dislocations in the sturcture, the structure is less likely to move but many of the atoms in it, are not where they'd actually like to be, blocked from moving by a grain boundary or some foreign atom. Internal stresses increase but the benefit is a harder stiffer casting. Think pre stressed concrete....!
if you make the iron an alloy, by adding in atoms of a different size they act like dowels in the structure they reduce movement but becasue the iron atoms can't slide past the foreign atom as easily, any working of the block makes it get stiffer faster. which is why blocks n heads are made from a specific version of cast iron or aluminium which is an alloy, not pure iron not pure aluminium. you have the stiffness from day 1
with your iron atoms all pined into position by the introduction of the foreign atoms in the alloy, anything that is stuck at a grain boundary or by another atom by its nature would rather be somewhere else. so there are internal stresses
if you freeze it you make all of the atoms move closer, its like filling a jar with rice and then tapping it until all the rice settles, and then you can get another 1/2 inch in at the top.
the atoms all slip into a resting place that they could not achive at room or operating temperature
they do this becasue they never sit still at room temperature, they vibrate about a position quite wildly
when cooled to near absolute zero they have less energy becasue we sucked out the heat energy that made them vibrate they vibrate a lot less and can hence slip through gaps in the structure or past the foreign atoms to reach that place of rest that they could not reach before.
It is easier to shoot the bulls eye when standing still. much harder if you are jumping and running about.
If you went to absolute zero all vibration stops and all movement stops so they don't go that far
when the metal is warmed again they vibrate much more again, and can't move back to where they were, but don't particularly want to, now they have got to the best most natural resting place, something that was unachiavble at room temperature. becasue they were buzzing about way too much to hit the target
Its another way of untangling and re tangling the structure so it gains the appropriate level of stiffness with the least internal stress, so its less likely to move in use.. i.e the block will "move" less as it ages. it achieves by other means that status of "seasoned" . A stable structure that will not change shape as much as a fresh new casting that has only known casting, room and eventually operating temperatures.
next time you see an old but high end record player/turntable with cast aluminium platter.. notice it has a slight wiggle...it didn't when made, but 50 years later that platter has moved, the kids really didn't drop something on it to bend the spindle...natural ageing caused that. if the platter had been cryo treated it would more than likley not have gained a slight variation in its 50 years existance.
i belive a race team will often dig out a well used block on a virgin bore to build a max effort race motor over a brand new casting... they know the new casting will move in use.
or they may spend money on getting it heated and cooled or they may get it cryo treated when they probably woudn't on the old block that has done its "movmenet stuff" whilst working for a living in a car or truck.
Dave