Analogue modulation for any audio source
The Tracer City is in many ways very similar to what you might find on an old (or new) analog synthesizer. The main components of a synthesizer it has are filters, an oscillator (which can also be used as an LFO) and a Psuedo-Random LFO with lag. It also has an envelope follower (with up or down response) with lag, which works very well with drums, loops, and anything else which is very percussive. Like a synthesizer, the Tracer City allows its multi-mode Filter to be controlled by two external CV sources. You can also plug audio sources into these inputs (this creates Frequency Modulation of the Filter or FM) If you are familiar with modular synthesizers or analog synths, then the control names in many cases should be self-explanatory and you will simply need to get a hang of the layout and get used to the way that the modulators are routed to the filter.
More Info: Snazzy FX Tracer City