I struggled with this for ages, and am still struggling to grasp the logic in these control systems. There is a master autopilot on/off switch on the dashboard, this activates the SAS, then there is the autopilot button on the stick, which turns autopilot on and off as needed (but you need autopilot master switch on to do anything). How that differs to the trim and mag brake is something thats still evading me.
The autopilot should self-engage at >120 kmh anyway, so what does the little autopilot sick button actually do, and how should it be used correctly? Its almost as if this whole helicopter was designed by French people ;)