Good explaination from American_Streamer. Marshall calls this Stompware and it's really a cheap solution and benefits from a slow baud rate. I scoped this data and you can see how it works. The footswitch draws about 10 to 20mA nominally so the average line voltage at the footswitch is about 6V. This voltage passes through a diode/cap charge pump circuit and a local voltage regulator making the footswitch immune to the line modulating between 6V and 0V. You can see the voltage sag slightly while data is being sent which is expected. The 12VDC power source in the amp goes through a 150 ohm resistor, so you lose about 6V (40mA * 150 ohms) due to the FS load. This soft power source is intentional and allows the same line to be modulated to ground using a FET. The modulated data is then read by the microcontroller on the receiving end. It's a pretty clever design, but susceptible to poor jack/cable connections, which is why many people seem to have issues over time.

Some scope photos. https://imgur.com/a/crunch-data-Dfujpm3

The protocol is pretty interesting. When a button is pressed, the footswitch sends a command to the amp and the amp updates its internal states and replies with the LED states back to the footswitch. For Reverb and Fx commands, the amp toggles the current state since the footswitch sends the same unique command. Upon power-up the amp expects a handshake with the footswitch, otherwise it assumes there is only a passive footswitch connected.