It disengages when you fight it. With the associated and very recognizable autopilot disengage sound. I'm sure there is more to this story than "crew mistake" as some seem to imply here.
To be fair, the "some seem to imply" is directly sourced from the analysis of a 777 pilot. Do you have a source for the "it disengages when you fight it" assertion? You're kinda calling the 777 pilot a liar ;-)