Also referred to as simply a Leslie, this is a (generally) bi-amplified speaker system for use with an electronic organ where there is a rotating baffle in front of the woofer and where the tweeter horn also rotates. The Leslie was introduced as an adjunct to the Hammond tonewheel organ.
The Leslie’s rotating parts provide an amplitude and frequency modulation to the sound in a manner that cannot be duplicated with stationary loudspeakers. The effect is a strong vibrato, where the frequency modulation is caused by Doppler shift, combined with tremolo caused by the woofer baffle and dispersion pattern of the horns, which are not always facing the listener. Add to that the sonic characteristics of the old tube amps that were used and the somewhat inefficient drivers and the Leslie stands out as having a very distinct sonic signature, especially when combined with a good organ (which may have many sonic idiosyncrasies of its own).