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. Bingo! The Leslie Speaker was born
The spinning horn gave the Hammond Organ what we know in the pipe organ world as 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.