If your oscillator can do TZFM you can bring it’s frequency down to zero, where it stops oscillating and then to “negative frequency” where it starts oscillating again, but with a reversed waveform.
This has two main usages:
first you can do normal FM sounds with very deep modulation without getting strange pitch shifts because the frequency is capped before zero.
Second, you can set the oscillator to start at zero and then modulate symmetrically around zero. The sound you get is quite unique — ranging from woody sounds to growling lead sounds. The fundamental pitch is decided by the modulator (as opposed to the carrier in usual FM). Symmetric TZFM takes the modulator signal and integrates it, then adds harmonic content by folding according to the waveshape of the modulator. This works best if the two oscillators are kept in soft or hard sync.
My favourite patch is to use a harmonic oscillator as modulator for a TZFM carrier. Modulating the distribution of sines in the modulator and the frequency of the carries gives such a unique set of timbre controls.