I spent a while playing with the new plugin this morning. It's both a harmonic enhancer and a harmonic synthesizer. The enhancer part works similarly to his other "saturation" features (see the free MLimiter) but with far greater flexibility. The synthesis part didn't strike me as anything I'd have a use for. Making weird chirping noises, perhaps.
The most interesting aspect is the ability to modulate harmonics. There's a preset called "slow character modulation" that'll give you an idea of what that's about. At least it makes for a cool visual, watching the harmonics go up and down.
Note that it only works on monophonic sources.