The Hammond M-series organ service manual describes the operation of Hammond's tonewheel generator this way:
Electrical impulses of various frequencies are produced in the "tone generator", which contains a number of "tone wheels" driven at predetermined speeds by a motor and gear arrangement. Each tone wheel is a steel disc similar to a gear, with high and low spots, or teeth, on its edge (see figure 12). As the wheel rotates, these teeth pass near a permanent magnet, and the resulting variations in the magnetic field induce a voltage in a coil wound on the magnet. This small voltage, when suitably filtered, produces one note of the musical scale, its pitch or frequency depending on the number of teeth passing the magnet each second.
The phrase, "when suitably filtered", is key here. Unlike those organs which produce complex tones and then use filtering to achieve the sounds of traditional instruments (subtractive synthesis), the Hammond organ produces nearly pure sine wave tones and then combines these to create instrument sounds (additive synthesis). The relative proportions in which these are combined are controlled by the organist using the Hammond organ's harmonic drawbars.