Ok I had this build a few years a go and I'm no longer using it.
Basically its a box that has 2 inputs that you can switch between and 2 outputs you can switch between or have both on at the same time and it has an fx loop between them.
Basically I had it build for biamping and when biamping you want some fx common to both amps such as boost and tuner so you can boost both or mute both without having to step on more than one button
It's the grey box incase you can't tell
It has 4 jacks on the top (Input/Output) and 2 on the side for the fx loop, inside the bottom of the unit tells you which jack is which since it wasn't labeled so it looked cooler
It's a very useful tool and very well built
Buttons are input a/b, y output, a/b output
I'm after £80 for it but usual offers and everything applies.
Cheers