The classical Leslie cabinet works as follows:
- Signal comes from the Hammond organ's preamp via a non-standardised multipole connector
- Signal is again amplified by an internal amplifier, which may be tube or solid state
- Signal is split at 800 Hz into feeds for the bass driver and the horn driver
- The horn driver is (usually) an 1" Jensen driver, emitting its sound upward into a Bakelite horn rotating in the horizontal plane. The rotor looks like it's made of two horns, but one of them is just a counterweight. In the horn's opening, there is a diffuser which is supposed to make turn the directional lobe of sound into something more complicated, adding more doppler effect. Many organists cut away the diffusers, though, as they hardly change anything about the sound, but make the horn more noisy when spinning.
- The bass driver is a large, well, bass speaker emitting its sound downward into a rotating scoop, also called the chute or the drum. It's basically a plywood cylinder open at the top and open at one side, with a curved plywood reflector inside that guides the sound from the top opening to the side opening.
- The Leslie has two speed settings: Chorale and Tremolo. At chorale speed, usually only the top rotor spins -- slowly, at about 48/min. At tremolo speed, the top horn spins up to a tremendous ca. 400/min and the bass rotor kicks in at about 340/min. Some Leslies do spin both rotors at both settings.
- Switching the Leslie is traditionally done by a halfmoon switch, that is, a small semicircular switchbox with a small "CHORALE/STOP/TREMOLO" lever mounted to the front of the organ. Rock organists, who usually don't play pedals, seem to prefer switching the Leslie with a footswitch, though.
- Classic Hammond organs feature a three-position "MAIN/ECHO/ENSEMBLE" switch. On MAIN, output goes to the organ's internal speakers; on ECHO, to the Leslie; on ENSEMBLE, to both. The terminology, like the labels "GRAND" and "SWELL" for the organ manuals, comes from pipe organ tradition.
The Leslie's sound is a complicated effect which cannot be defined in terms of chorus, vibrato, phasing, tremolo or reverb, especially because the Leslie is, unlike most other effects, spatial -- even a stereo recording can't do justice to the sound a Leslie can fill a room with. Recording a Leslie's sound can be a daunting task, requiring up to four microphones.
To play a Leslie with a P.A. system, many bands have chosen using the Leslie in an effect loop, that is, putting it backstage into the wardrobe --or, ideally, in a bathroom, as hard, reflective walls increase the effect-- and then taking the sound back to the mixing board with microphones. Legend has it that obnoxious Led Zeppelin fans once went backstage to find John Paul Jones' Leslie happily spinning away in the wardrobe and misusing the microphones to feed their personal messages to the P.A. ;)
There are many Leslie simulators and clones around. Many of the clones sound quite good, such as my old Allsound LC 80 / TH 85 cabinet or the much-praised Motion Sound Pro3T / Low Pro combination. The simulators usually don't rate good in comparison with anything that really spins, but yes, Jon Lord of Deep Purple fame uses a H&K Rotosphere from time to time.