Just FYI, here are two examples of " trip strips " one on a new Fiesta the other on a new Focus .
One is a vertical break just before curve at the rear of the car , and the other is a ramp style which snags some of the air that has already been detached.
You can easily make your own by using some flexible wire and some electrical tape ( high quality electrical tape won't leave the sticky residue that cheaper tape will. In black it also shows up less )
Here is the clean break ( trip ) line on the Volt as seen from the rear :
( The vertical line just below the taillights )
From what I understand, a vertical 'cut' line just before any aggressive curving at the rear of the car acts to form a clean separation point . Trip strips perform the same function that a spoiler does , but for the sides of the car . That's my understanding at least.