I suspect the advertising hype is probably the opposite of what happens. Any head fired at game should be sharp enough to shave with, and will bite into green bone (softer and more easily cut than old or cooked bones) If the head is firmly attached to a spining shaft it has some force turning it to new positions. If the head is free to spin then it will be guided by the edges and drive through the bone. I would not suggest that driving through bone is a bad thing, but the reason most "stoping power" theories fail is due to the number of variables involved in any projectile / target meeting. This looks like one more unecssesary variable - will it spin or stop?
Having said that I would be interested in observations of anyone who is using them, and like Havius idea of slow motion film.