This is NOT a simulation of a Hammond, which is actually pretty complicated. The sound is "just" a stored-wave oscillator with 8 harmonics specified, that is reminiscent of a Hammond with all drawbars out--but in some ways quite different from the real thing.
Instead the interest of this patch is that it uses a Leslie Speaker "package". In this patch, the Leslie looks like the built-in modules, but in fact it's a "mini-program" (what programmers call a library) that contains within it several basic Moselle components wired together. These include a delay line, two LFOs, two Slew modules, and a crossover filter. The result is a simulation of the low and high frequency rotors, each with independent low speeds, hi speeds, speed-up times and slow-down times. The General1 controller simply chooses whether its low-speed mode or high-speed mode: a switch, like a real Leslie.
There's actually a waveshaping distortion package used as well, which is cranked up half-way.
Note the sound is recorded directly from Moselle: no hardware or external processing was used.