There is some physics involved, if you take warm air and spin it round real quick, the hotter air gets thrown to the outside of the tube, and the colder air goes down the middle. wether this partially separated stream of air makes any difference to power Ive no idea. Id be interested to see exactly how it works, and if any gains are possible.
My doubts are, that in this world where fuel efficiency and power are chased by manufacturers, and millions of yen spent on research, does a little vane thing stuck in the exhaust make any real difference?
Why dont manufacturers fit one?