This interweb thing is awesome :P
Here's an excerpt of how it works from his website: A motor controller and electric motor rotate a set of blades at a constant speed. The TRW-17 Rotary Woofer pitch mechanism uses a conventional voice coil and magnet assembly. This is connected to your amplifier to pitch blades in proportion to the applied audio signal. As the blades pitch while rotating a pressure wave is generated, the degree of pitch controls the amplitude of the pressure wave. Air is allowed to transition through the blades. Thus oscillating the pitch of the blades creates sound while they are rotating .
