My understanding may be incorrect, after all the freezing the drives trick is mostly something that was shared among sysadmins like an old wive's tale. Nonetheless, it does work (or did).

The way I had always understood it was that if you had a head crash (which was caused by the head physically contacting the platter, overcoming the resistance of the air gap between the head and platter, usually due to physical impact) that the magnetism of the head would prevent it from lifting back up on its own, and that freezing the drive would cause the metal in the platter to contract away from the head, which was restricted in movement by its armature, thereby restoring the air gap and lifting the head away. If you started the drive spinning before it heated, the head would stay out of contact and you could successfully read data (some of it, for awhile).

That said, my understanding may be incorrect.