A rotary cabinet puts a spinning horn on top of a spinning drum and points a microphone at each. Because the drivers move toward and away from the microphone, every rotation shifts the pitch slightly and the level with it. That combination is what a plain tremolo cannot reproduce.
Rate
Rate sets the horn speed from 0.1 to 8 Hz. The drum runs at 78% of it, so the two halves drift in and out of alignment instead of pulsing together.
Original cabinets switch between a slow and a fast setting rather than sweeping, and the sound of them speeding up or slowing down is half of what people recognise. Moving the rate control during playback ramps rather than jumping.
Depth
Depth scales both the pitch swing and the level swing, from 0 to 100%. They are not separate controls because they are not separate effects on a real cabinet: the same rotation causes both.
At low settings the result is a gentle wobble. At 100% it is the fully modulated sound, with the horn’s level dropping by half as it turns away.
Stereo width
Stereo width offsets the rotation between channels by up to half a turn, which is where the two microphones of a stereo cabinet recording sit.
At 100% the horn is arriving at one side as it leaves the other. At 0 both channels move together, which sounds closer to a single microphone in front of the cabinet.
Mix
Mix blends the cabinet against the original signal, from 0 to 100%.
Full wet is normal for organ. On guitar or electric piano, 60 to 80% keeps the direct sound in front and puts the rotation behind it.
Example: an organ part on the fast setting
Rate 6 Hz, depth 80%, stereo width 100%, mix 100%.
The horn swirl sits over a bass that stays put, and the offset between channels keeps the image moving rather than pulsing in place.