Basically, you alter the car with not-licensed-for-street-usage components (these self-made things e.g. alter the ride height of your car from the one spec'd in the vehicle ID card), which in itself invalidates the car's insurance (it's not a vehicle legal to drive on a street, thus it is not insured, thus you are not allowed to drive it!).
It's the same as if you'd change the exhaust for an illegal tuning variant, add a N2O booster, add under-floor lighting etc. DO NOT EVER DO THIS IN ANY EUROPEAN COUNTRY.