The way they did it was elegant.

They knew that when doing the testing, the car would be on a rolling road inside, and usually when they do that they have the car doors open to let air go into the cabin and to talk to people etc.

So they programmed in a rule that "if the car is doing 100kph, and the door is open, change engine map to use more fuel and output less emissions".

Because it was extremely unlikely someone would have the car door open and doing 100kph in any other circumstances.

The red line is the Euro6 emissions standard.