The making of “ice” beers, in general, involves lowering the temperature of the finished product until the water in it begins to freeze and then filtering out the ice crystals that form. Since water will freeze before alcohol, the result is higher alcohol content. The ice forms around yeast cells, protein particles, etc. so these get removed as well; leaving fewer components to provide taste and character.
This process is not new to brewing, having been developed in Germany to produce “eisbocks”. Apparently they were produced by accident during the traditional spring celebration with bock beers. Spring, being the capricious season that it is, probably sent a late cold snap around one year causing some of the spring bocks to partially freeze. People drank it anyway and liked the change in flavour.