Using multiple selection
You may find at some point that you want to add another small area onto your current selection or remove af few pixels without having to reselect the whole thing. Pixen lets you do both of these things through the modifier keys.
To add to the selection:
Hold the shift key while working with any of the selection tools, and what you select will be added to the canvas's current selection.
To subtract from the selection:
Hold the option key while working with any of the selection tools, and what you select will be removed from the canvas's current selection.