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The reason that this works, is that when a head crashes it gets off center and will hit the pin the harddrive. Freezing it compresses it, allowing it to gain a little room to move where it wouldn't before.
This really works if you catch the drive before it gets too bad. I used to work desktop support and we would leave the hard drive in the freezer over night. In the morning we would put it back in the pc or laptop and take an image. It was always funny to see a couple of harddrives mixed in with the frozen waffles.