In the mid-20th century, Mr. Leslie was disappointed by the sound of his newly purchased Hammond organ compared to what he had heard in the store's large showroom. He then tried to design a system that would give the same feel as a pipe organ through a moving speaker. Other experiments resulted in the familiar combination of a rotating drum and a horn for low and high frequencies respectively, creating special audio effects using the Doppler effect.

Both the horns and a sound deflector for the bass rotate to create vibrato, tremolo, and chorus effects based on the Doppler effect. The rotating elements can be stopped, switched between slow (chorus) and fast (tremolo), or transition between the two settings. Not only does the unit produce this particular effect, but also the ensemble it creates with the room in which it is located.

The internal reflections of the cabinet are also taken into account, which give that familiar mellow sound, but with the never-before-seen possibility of increasing or decreasing their effects, going from a standard closed cabinet to open-air rotating speakers.