system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is a “defeat device”, and how does it work?

I’ve just started watching Netflix’s Dirty Money, and the first episode on Volkswagen’s emissions scandal revolves around defeat devices, how the NGO that discovered the lies couldn’t believe at first that the company had put in place such a device, and that while tests showed that the cars complied to emissions standards in the labs, the same can’t be said when they were tested on roads (I hope I’m right). I just need to wrap my head around the concept. Thanks!
gpt
In this case, it's just a device that senses when an emission test is taking place. When it senses the test is taking place, it alters how the car runs to conform with emissions requirements. In practice, this usually makes the car accelerate less aggressively. So when it isn't sensing that a test is taking place, it runs in a way that does not conform with emissions standards or even its own test result.

It's a way of gaming the emissions testing system.