When you turn the Leveler knob anti-clockwise from the "OFF Position" you will be enabling the leveler's function whose characteristic is similar to an audio compressor with both a slow attack time e.g. 100 ms and slow release time e.g. 1 second (or 1000 ms).
Gain-reduction will be applied gently such that the leveler will not react to fast transients due to the slow attack time, and pumping effects will be reduced because the Leveler has a slow release time to address that.