Dynamics Plug-Ins : Channel Strip : Sections and Panes : Listen Mode

Listen Mode
The Side Chain tab in the Dynamics section, and the EQ and Filter tabs in the EQ/Filter section provide a Listen button.
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When enabled for the Side Chain, Listen mode lets you hear the input signal that feeds the dynamic section. This can be either the external key input or the internal side chain (including the applied filter).
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When enabled for any of the EQ bands, Listen solos the corresponding EQ band and (temporarily) inverts the EQ Type so that you can tune the Frequency and the Q for that EQ band.
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When enabled for either of the Filter effects, Listen solos the enabled Filter band and inverts the Filter so that you can hear the audio signal being fed into the filter.
To enable (or disable) Listen on the Side Chain effect, EQ band, or a Filter effect:
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Click the Listen button for the Dynamics or EQ/Filter tab you want so that it is highlighted. Click it again so that it is not highlighted to disable it.
 
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Control-Shift-click (Mac) or Start-Shift-click (Windows) and hold an EQ or Filter control point in the Frequency Graph to temporarily switch to Listen mode for that EQ band or Filter effect.