In HF mode with keytracking you simply have a third oszillator and by using the DELAY slider some kind of minimalistic wavemorphing.
In HF mode without keytracking you could use it like the fixed pipes (drones) of a bagpipe.
Another interesting patch idea is to use it as a very deep sawtooth that stimulates the filter resonance and than play the filter with keytracking.
An easy example for that:
p6_PlayTheFilter.s6 (35.4 KB)
… with this settings:
You can try to use other waveforms (see Waveform and Spectrum visualization to see how they look), but I think the effect of the melodic filter ringing comes best with a simple sawtooth.
Some other synths have a function to disable keytracking for regular VCOs what results in the same effect.