This is a very, very simple drone set up. It can easily be expanded upon by the modular savvy but as is has some range of expression. Simple is good. 

Knobs to turn are...

On FM's: all of them but the feedback and FM knobs(on both flip sides) are esp. expressive. 

On LFO: all knobs. 

Why is this cool? * if you like drones* Because it feeds into itself, drones endlessly and is elegant in design. It is also wide open for additions.