A knockdown is achieved once you cause another racer to crash. You can either push another racer's car towards the wall, to make him crash, or hit their car with a lot of force( drive a lot faster than they do). If you have nitro, I prefer the second option. Drive behind a car and use nitro, preferably perfect nitro. If you close in fast, on the other car, and hit the car after, it will knock their car down. You have to do this on 25 cars, can be the same car, and it is cumulative across all races and seasons.