The envelope follower modulation source will output an envelope similar to the side-chain signal. The amplitude of the positive peaks of the input signal is measured and the outer shape (= envelope) is the output signal of the modulation source. You can set the Attack and Release parameters to 'smooth out the bumps'.
To add an envelope follower as a modulation source, click the + button in the source selection bar and click New Envelope Follower.
The two buttons at the top of the EF source interface select which signal is used to trigger on: the main input signal or the signal from the side-chain input. See also VST plug-in versions.
At the top right of the source interface, the Presets button provides access to the EF section presets. The Remove button deletes the envelope follower. You can customize the default EF settings (used when creating a new EF) by overwriting the predefined Default section preset.
Next: MIDI source
Drag-and-drop modulation slots