It overrides the EGR / DEF emission functions. You take off the factory exhaust and straight pipe them and get rid of all those filters that in essence choke the engine. You no longer need to use the diesel exhaust fluid and get a programmer, you can set your HP needs to your application whether it's towing a heavy load or not. It's highly illegal especially in the not so great state of California, where they have more people living in one city then we have in 4 states combined :Rofl