Schematic is available on the "Building guides" page in DOWNLOADS area (you have to be logged in).
The board provides 2 x unbalanced RCA and 2 x balanced XLR inputs, converting them to 2 x unbalanced outputs.
Control is very versatile! Depending on part and jumper configuration it can be switched manually from a rear panel selector in hardware mode or via software over I2C. In software mode, it can use either a latching or momentary relay. Rear panel switch can be read via I2C as another input option. Digital voltage can be 5V or 3V3 be selecting the appropriate relay to match. In software mode, the relay is powered from the I2C connection. In hardware mode, it's powered from one of the analog supplies using a 24V relay and current limiting resistor.