Don Leslie capitalized on a principle called the "Doppler Effect". It's why a train whistle seems to go down in pitch when a train passes you. Don took a horn speaker and made it go around in a circle. And Bingo! a legend was born.
The spinning horn gave the Hammond Organ what is called in the pipe organ world a tremulant. Vibrato, Tremolo, and a little "whoosh" of air all put together. It made the Hammond Organ sound completely different. The Hammond's life in all popular music - Jazz, Rock, Pop was born at that moment.
