The “+/-” button determines whether a signal is triggered on ascending flank (+) or descending flank (-). If the button is off, the ascending flank is used, otherwise the descending flank is used. This setting is independet from manual or automatic trigger level setting (independent from the “At/Norm.” button and the “Level” wheel).
The following images show examples for different trigger settings:
The “At/Norm.” button is off (=automatic level selection),
the “+/-” button is off (=trigger on ascending flank)
The “At/Norm.” button is on (=manual trigger level setting),
the “Level” wheel is turned to a position lower than the lowest voltage of the signal,
therefor the signal cannot be triggered, the screen remains empty.