REF allows you to modulate anything you want in Ableton Live: other devices, the user interface, and third party plug-ins, using sound as source.

You can shape the incoming audio source with the low/high pass filter, listening the source in peakor rms modes, then design the envelope exactly how you need it, using tools as relative/absolute modes, linear/logarithmic modulation, and positive/negative directions. You can even work on your envelopes with standard EG parameters as attack, hold, release, and a global delay time.

REF uses a big display to give you an accurate visual feedback: it will help you to check the incoming audio signal, set the threshold level, and view the real envelope generated, which will control the chosen target parameter.

REF is a great tool for any kind of modulation: kick/bass side-chain combinations, driving filters, and experimental uses asking for controlling parameters from an audio source.