What does it mean when a race engine noses over?

That is a slang term for the motor going outside of its power band, thus stops pulling, or accelerating. So, the car goes from a couple of degrees of negative G forces, to none, which is equal to hitting the brakes, hard, and sudden. No real toll to the driver, however, in racing, if you are not accelerating, you are losing. Another way to say it is it is "laying down."