@oblomov By default it triggers a function in the keyboard firmware that produces a random printable character. I've not looked at the detail in depth, though the firmware source code is available. See https://shop.keyboard.io/pages/code for links to the code.
If @obra or @algernon aren't busy, they may confirm or correct me.
@algernon @liw @oblomov Specifically https://github.com/keyboardio/Model01-Firmware/blob/master/Model01-Firmware.ino#L296 - It's 'Key_A' plus the number of milliseconds since your keyboard's been on, mod 36.
Lars and friends