You can get better control of your Envelope shapes by adding extra nodes.
1. Select the Envelope segment you wish to edit
2. Activate the Shape Tool (Toolbox > Shape tool or F10) and right click on the segment.
3. Choose Add (or Delete) for extra nodes.

As an alternative method:
Left click the segment and press Plus Sign ( + ) in the numerical keypad.

Tip provided by Silvio Gomes, CorelDRAW Master, Artist and Graphic Designer.