This is a very useful tool for bass and guitar. It’s a footswitchable effects loop with blend control and a phase reversal switch.
If you’re a bass player it allows you to use guitar distortion pedals by mixing effects in the loop with clean bass signal. This way the crucial lower frequencies of bass won’t get lost. Blend is continuously variable to find the perfect mix and if effect in the loop is inverting phase you can reverse it with the flick of a switch.
For guitar use you can for example mix compressor signal with clean for nice transparent compression. Another good way which I like is mixing two different kinds of fuzzes/overdrives/distortions to create complex and cool distortion textures!
If you’re wondering about the name it was a custom 40th birthday build… :)