The job of the exhaust gas is to take up space in the combustion chamber. If there is no EGR there, the cylinder would fill up with alot more 02 and therefore causing a bigger bang witch equals to more heat, creating NOx. When the exhaust goes back into the intake manifold through that little hole, it cools down quite a bit and would take up enough airspace so where the combustion chamber temps dont get as high to where it creates NOx. Remember, this mainly happens when you are cruising down the hwy at a light load on the motor, so it wont really effect your WOT performacne.