It is explained in the article, they developed software that knew when it was being tested and then changed its tuning to meet the testing requirements, tuning

It is explained in the article, they developed software that knew when it was being tested and then changed its tuning to meet the testing requirements, tuning that would never have provided the performance and characteristics owners expected when driving the vehicle. Those in charge knew it was being done and approved it. Hence the broke the law and off to jail. I lost my TDI because of this fraud as VW wasn't going to have an acceptable correction available and while I could have kept it outside of others who just enjoyed TDI my ability to sell it later would have resulted in vastly reduced value.