Servo test function in CLI -what's the actual syntax?

I am running a hacked version of Plane in SITL mode to test out new ideas for my VTOL design with X-plane 11 but I’d like to use the servo test function in the CLI. The usage description is most unhelpful and grep isn’t telling me where in the codebase this is all going on. Can anyone post me a clue? Thanks in advance!