I have an older diesel VW. It is extremely common for people to run tunes that disable the check engine light that is produced when you delete the exhaust gas recirculation valve (EGR), urea pump and diesel particulate filter (DPF).

For things like the EGR, blocking it off is simply installing a bracket or two and closing off some vacuum lines. I can imagine people would simply reinstall this and swap the tune to pass and then immediately remove it after.

In the US, for emissions, all you have to do is not run any check engine lights when at the test center which is easy to do in the software tune.