agreed with all the above..including the ones who said "not frozen ALL the way down." As slow as their metabolism gets in the cold, they still have a little: swim bladder still works, gills flap however infrequently etc. This generates a tiny amount of heat. And they bunch together in the deepest/warmest part of the enclosure which amplifies the effect. so they can be in a little tiny bit of not-quite-frozen-solid water which is very hard to detect.
that's my theory anyway and I'm sticking to it.
Have had what started out as 6 feeders in an outdoor pond in w.TN for 11 years now and admittedly I have never checked depth of ice once it freezes over (which it doesn't do every year, it is TN after all.) But have never put more fish in and have dozens now. Including some who have clearly lived over from year to year, they didn't just hatch over winter from eggs--one of the originals I called Don Diego lasted until around 2008 and I think it took a raccoon to do him in. Although I get babies every spring as well so some eggs do survive as well.