it shows that the flow is attached which means there is no unnecessary drag but it does show a sharp down flow on the second row of tufts which is probably not good, wouldn't be surprised if they add some turning veins on the side pod to stop the downflow
yah air flows around and down... just following the path to lowest pressure. Diffuser and any rear underbody generally is low pressure. Flow always separates around the coke bottle, you need a way more streamlined shape to stay laminar through there.
Unfortunately, from a single still image, you can't tell much except a general trend.
I did this exact test on my car a few months back with a chase vehicle and video camera. When there is turbulent air and/or separation, a still image will only show a portion of the full truth. One moment the tufts could be pointed down and the next they might be pointed up, forward or back. I could actually "measure" the amount of turbulence for each tuft by the amount of fraying experienced at their ends from all the flapping around.
