Promax OD-610 User Manual page 23

PS:
Press the button "50%" to set the trigger level as the vertical mid point
values of the amplitude of the trigger signal.
Press the "Force" button to force a trigger signal, which is mainly applied
to the "Normal" and "Single" trigger modes.
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Turning the TRIG LEVEL knob can change trigger level value and
it is also the hotkey to set trigger level back to 0.
June 2017