The reason for this: The autopilot normally uses very minor angles of the helm while maintaining the set course by the OOW. Cases have been known in the past where the steering did not respond to sudden, large helm orders while using the autopilot in the manual mode, in a matter of urgency.
The test: Ensure that there are no ships or dangers nearby. Switch the steering system to the manual mode. Put the helm over to about 15° to one side.
Then put the helm over to about 15° to the other side.
See if the rudder indicator shows the appropriate rudder angle each time. A significant alteration of course is not necessary.