After doing a little research, I am thinking that those flaps, when partially open might be used to increase the velocity of the exhaust causing the turbos to spool up faster.
The gas would be faster as it passes through the restriction because that's how fluid dynamics work, but that does not help the turbos spool faster. The only thing that will help the turbos spool faster it a bigger pressure difference from one side of the turbine to the other. The lower the pressure on the outlet side of the turbine and the higher the pressure on the inlet side, the faster the gas will move through the turbine, thereby spooling it faster. You could weld a washer in the tailpipe with a 1/4 hole in it and the gas would be moving insanely fast to squeeze through the hole, but it's causing a massive restriction, thereby decreasing the difference in pressure between the inlet and outlet sides of the turbine. The faster you can get the gasses through the turbine to the other side the better. Creating a bottleneck post turbo is bad for performance. That's why chambered mufflers are frowned upon in the turbo world. The only reason people run any muffler at all is because the noise would be intolerable otherwise.
The backpressure debate is a very misunderstood concept. Cars don't need backpressure. That is a myth. The reason people think having no backpressure causes your engine to burn itself up is because cars that cant self correct their fuel will run super lean if they aren't tuned for the increased airflow. What NA cars DO need is exhaust gas velocity so the gas has momentum for the purpose of scavenging. The way you get velocity is to run smaller diameter pipes, not by creating a cork. Smaller pipes=more scavinging=more low end power. Bigger pipes=more flow=more high end power. Backpressure is a side effect of the smaller diameter pipes. It is not what gives you more low end power.
Forced induction engines don't care about scavenging because their intake air is getting pushed in during the valve overlap rather than being pulled out by exhaust gasses.
As far as I can tell, the flap is there strictly for noise reduction, and it does a helluva good job at that.
edit: and for cold starts maybe.