This is a feedback loop device into a wah pedal enclosure, which allows for an intuitive control over your feedback-noise performances, especially if you’re already using your hands for other things (say, playing guitar or keyboards, or even twiddling with other knobs). As you can see, it is useful for making a wide variety of high-pitched screaming sounds, and bringing out the unexpected from effects pedals and other devices.
It was a fairly simple project: I used a schematic from Beavis Audio Research, though I replaced the 500k pot with the 100k I found in the wah pedal.
Feedback loop controllers like this one do make very interesting things happen — however, always keep in mind that you’ll need the feedback loop you’re controlling to add gain. If you have less than unity gain, you’ll just find that the loop gets very quiet when you open it up all the way.