When you make a movie, a code is inserted into the movie to say which codec you have used to make the movie. This is called the FOURCC code. When you play back a movie, the FOURCC code is looked up to see which codec (decompressor) should be used. What the chap says is: If you decide to encode a movie in 2 parts and have found that one codec gives better quality for the first half, and another codec gives better quality for the second half, you can make the movie using the 2 different codecs. Then you can change the FOURCC codes in the movie files to use the same decompressor. This only works because many codecs support decompression of other codecs, so changing the FOURCC code to another codec which supports your original codec you encoded with should work fine. All in all, a lot of faff for very little benefit. I've never had the need to do this. There is also another benefit of this tool: If you can't get a codec for the FOURCC a movie is encoded with, try changing the FOURCC code with this tool to another codec and maybe it will play !