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The basic Hammond tonewheel timbre is rather static, and becomes uninteresting to listen to after a short time. Hammond realized this early on and undertook development efforts that eventually produced the vibrato scanner. However, Leslie came up with the idea of the rotating speaker, based on some work that he had done while working for the U.S. Navy's Naval Research Lab. He tried to sell the idea to Hammond, but they rejected it.
So Leslie formed his own company, named Electro Music. The first Leslie speakers went into production in 1941, but production was limited by WWII until 1946. After that, Leslie began supplying models to notable Hammond organ players, and soon public demand built up by word of mouth. Electro Music worked with Hammond dealers to include Leslies with organ sales. Dealers had to be careful because Hammond sold its own speaker cabinets (few of which used any type of rotating speaker), and they regarded Electro Music as competition. Many a Hammond organ was sold with Hammond's least expensive speaker cabinet, which the customer immediately returned in trade for a discreetly delivered Leslie.
Don Leslie sold Electro Music to CBS Musical Instruments in 1965. The company expanded their offerings to units meant for other organs, and "combo" units which could be used with other instruments such as a guitar or combo organ. Hammond finally purchased the company from CBS in 1980, and begain offering Leslies bundled with its organs. Today, both the Hammond and Leslie names are owned by Hammond/Suzuki.
The canonical Leslie model, the 122, contains a tube amplifier and two speakers, a tweeter and a woofer, both fed from a passive crossover. The speaker elements themselves do not actually rotate. The tweeter is a compression driver that fires into a Y-shaped rotating horn, one side of which is blocked. The woofer fires into the top of a cylinder-shaped wooden rotor which has a window cut in one side. Each of the two rotating elements is driven by a two-speed motor. In Leslie terms, the fast speed is "tremolo" and the slow speed is "chorale"; tremolo is typically around 250 revolutions per minute, and chorale is 30-50 RPM. Either a foot swtich or a "half moon", installed on the front edge of an organ and activated by the knee, is used to change speeds.
The rotating elements produce both frequency modulation and amplitude modulation effects as they rotate, due to the Doppler effet and the effect of the rotors alternately pointing towards and away from the listener. The two elements are not synchronized, so the resulting effect is complex. And, because the rotating elements have angular momentum, speed changes are not instantaneous; acceleration and deceleration are gradual, and the heavy bass rotor take more time than the lighter treble horn. Speed changing for the inertia effects is part of the technique of playing an organ with a Leslie. In addition, some performers set up their Hammonds so that at full volume it overdrives the amplifier in the Leslie, producing a variety of distorted growling and sweeping effects.
Connecting a Hammond to a Leslie is tricky and can be dangerous. Hammond used a number of multi-pin connector schemes across its organ line, and in some of these line voltage or tube B+ (plate) voltage is carried on the cable between the organ and the speaker. There are Web sites that specialize in matching up Hammond and Leslie models, and specifying the proper cables and adaptors to use.