Just like the original hardware, it allowed for complex FM-style modulation.
It was discontinued years ago. There is no legal free download available. Any websites offering "cracked," "pirated," or "free" downloads of Pro-53 are: Native Instruments Pro 53 Vst Free Download
Some legacy software archives host it, but these are often technically 32-bit installers that won't work on modern Macs. Just like the original hardware, it allowed for