Temeraire opened his jaws and what emerged was a roar that was less a sound than a force, a terrible wave of noise so vast it seemed to distort the air before him. (HMD, 330)
The Divine Wind is the most mysterious of dragon abilities-- a sort of 'sonic roar' capable of doing great destruction, especially to objects made of wood. It only presents itself in one breed in all the world, the Celestial. There are few Celestials in the world, and few of them ever venture outside of China, so very little is known of this ability in the western world. Sir Edward Howe suggests in his book Observations on the Order Draconia in Europe, with Notes on the Oriental Breeds (excerpted in HMD) that since the divine wind is known only by "heresay" and that some experts dismiss it as "phantasy."
Howe is naturally delighted to come into contact with Temeraire and confirm the divine wind's existence to other draconic scholars.