Hammond tonewheel organs have a row of spinning wheels with notches in them. The wheels spin over magnetic pickups just like an electric guitar string vibrating over its pickup. The number of notches and speed of the wheel determines the frequency that will be generated. The drawbars above the keyboard select the amplitudes of the fundamental and overtones that will be mixed to create any given tone.
The Leslie speaker was invented by Don Leslie. Hammond was not interested in selling the speaker but Leslie's spinning speakers and robust tube amplifier arrangement became very popular anyway. The Leslie provides the swirling combined tremolo and vibrato sound and can go fast or slow. Gospel, jazz, and classic rock bands such as The Allman Brothers Band, Boston, and Steppenwolf, to name a few of many, are famous for speeding and slowing the Leslie at just the right times to accent the music.
A 1973 Hammond B-3 and Leslie speaker
A close up of the key board. The reverse color keys activate preset drawbar combinations and do not play. Note the two switches on the top right. One activates a regular induction motor to bring the tone wheels up to speed then the other starts the synchronous motor and the tube circuits.
The inside of the B-3.
The preamp section is in the foreground. Behind that is the tone wheel generator spanning across most of the width of the cabinet. The synchronous motor is at the left end of the tone wheel generator. The generator in a B-3 contains 93 tone wheels. You can see many of the 93 corresponding solder terminals on the top of the generator as well as some of the magnetic pickups for each wheel sticking out near the generator's right end. The rest are hidden behind the preamp. The mess of wires in the top right is the patch panel for the preset keys and one can program them by arranging the wires on the rows of screw terminals. Across the very top edge of the photo you can see the back side of the drawbars.
The inside of the Leslie.
Note that the only one of the spinning horns in the top section is real. The other is a dummy for balance. The bass speaker points downward and a baffle inside the rotating black cylinder in the bottom section reflects the sound. The 40 watt tube power amp is at the bottom left and really makes the organ scream. The other the two chunky things in the center section are the motor assemblies.
The result of spinning speakers is both vibrato or FM caused by Doppler effect as well as tremolo or AM caused by the speaker pointing towards or away from the listener as it spins.
B-3's have two drawbars on the console to control the sound of bass notes played on the pedal board.