I think they realised that the outer edge of hte skirt was flexing too much and it was causing downforce problems as the ground effect essentially pushed the skirt up and broke the air seal... I think they reinforced the edges and other things as well as underfloor changes and it seems that this has kept the floor airflow intact... Now we've had relatively smooth tracks (although Hungary is a bit rough) and this will make their bouncing/purpoising better but we'll see in places like Singapore and other street tracks if they have truly fixed the problem... they're certainly a lot faster and you can see both drivers now being able to push in areas they couldn't before... however the tyre warming is an issue still and they're quite draggy too.