Has anyone actually seen one of these, or perhaps even played with one? http://www.patentstorm.us/patents/6623383/description.html It's mostly talking about tennis rackets, but I think it would lend itself to badminton much better. Basically, it means having a weight which can slide freely down the length of the rackets shaft. The racket up defensively, the weight slides to the handle (by gravity), making the racket head-light and more maneuvrable. When smashing or clearing, the weight slides to the head (by centrifugal force), making the racket head-heavy and more powerful. It sounds like a neat idea - I'm tempted to modify one of my old rackets just to see how/if it works
Sotx did the AK-series with sliding beads or sands inside the shaft: http://www.badmintoncentral.com/forums/showthread.php?t=34889&highlight=sotx+ak The racket would be a pain to play with though, due to constantly changing BP.
