Double-click on the envelope to create a new node where you want the swell to begin, and do it again to mark the end point

Bit - in X1 you don't even have to do this.

Just highlight a section of the clip and drag the highlighted section with the mouse up/down and that section of the envelope will go up/down with 2 pairs of nodes placed at either end of your selected area.

The only caveat is you must be in envelope mode via the edit filter.