Peak Rider 1.1

Been playing with this for a couple months now and still can't decide if it's a must-have or just a novelty. It's definitely something for the audio nerd who a) can visualize what it does and b) enjoys exotic automated track interplay.
 
Peak Rider is like a pantograph for audio levels. The audio follows a fixed ratio to the sidechain signal. For example, you can set it up such that one track is always 3 dB quieter (or louder) than a reference track. Like a compressor, the sidechain can be internal or external. When it uses itself as the reference, you get either an expander or a compressor.
 
Most of the examples the developer gives are using it as a self-referenced expander. Because you can filter the sidechain, it's possible, for instance, to boost just the kick drum embedded in a stereo drum mix.
 
You could also use it to keep a vocal a set interval louder than the instruments, like Waves' Vocal Rider, but such techniques don't appeal to me because they don't sound natural. The ratio of instruments to vocals is rarely flat across a dynamic song, e.g. the verse may be much more up-front than the chorus.
 
I see its greatest potential in level-matching multiple vocal tracks. But it's frustratingly non-simple to do this.
 
Like I said, it's for nerds.