How does supercooling beer work?

This friend would put a bottle of beer in a freezer for a certain length of time - ALMOST long enough to freeze it, but not quite. He'd get it very, very cold but it would stay liquid, and not slushy or hard. Then he'd take it out, very carefully open it - this is a glass bottle we're talking here - and then, with the bottom of a second bottle, give it a good rap on the top - straight down on the mouth of the almost-frozen bottle. What happened next seemed to be magic - in the course of 2-3 seconds, a cross-hatched crystalline lattice of frozen beer would form inside the frozen bottle, from the bottom up, and you could see the crystals reaching up and once they reached the top the thing would be frozen solid. Not just slushy, but solid. I assume there was some weird kinetic reaction that caused this to occur, but for the life of me I don't understand how. Can anyone explain?