AFAICR, H2O molecules are mobile in ice until -19ºC, that is they can move around albeit slowly, join together over time and form crystals which damage cells & tissues. Mobility becomes severely limited from -19 and below. So if you can get to -19 really quick then little or no damage will be done. That's easier for small things.

Incidentally, that's generally why food from the freezer tastes more bland if you leave it in there for months. Unless you have a decent freezer that goes to a bit below -19.