How can I mod a pedal (either phaser or distortion) to have a footswitchable feedback loop like in this video?
My best guess is that he just wired in a super high gain circuit in (possibly with a delay) and that just makes feedback naturally. I would just pm his YouTube.
Look up the Big Muff Feedback Loop mod. I'm willing to bet that the circuit is as similar as the casing they put the pedal in.