Your browser does not seem to support JavaScript. As a result, your viewing experience will be diminished, and you have been placed in read-only mode.
Please download a browser that supports JavaScript, or enable it if it's disabled (i.e. NoScript).
basically I'm too cheap to buy it and want to make my own version
my first guess is it's using the gain reduction of a compressor or an envelope follower to automate the volume, possibly with some other shenanigans?