confirms it. And it is the same as the youtube video above. Sure there are other kinds in the Musical Offering, but the crab cannon is simply what I described. If you take any two part piece and pull the bottom part off and reverse it and append it to the first part, this is what you will get. Then if you play it forwards while playing it simultaneously backwards, you will get the original piece followed by the original piece played in reverse. Playing the original piece in reverse may not sound quite as interesting as forwards but in general it will sound coherent. Try it.