The clear-eyed girl, sitting light and erect on the couch.
What does to sit light and erect mean?
Beachboy, imagine yourself sitting on a couch while watching a soccer game on TV. I bet you would not be 'sitting light and erect'. You would be comfortably sprawled-out while snacking on your favorite food and beverage! You probably would not want to invite the 'clear-eyed girl sitting light and erect' to watch the game with you!