According to people in attendance, both tires on the car's driver's side were torn off, and the impact ripped the master cylinder from the firewall. The throttle cable was attached to the cylinder, so when the cylinder fell, the throttle controlling a 1,500-plus-horsepower engine got stuck wide open for more than a minute. Worst of all, the initial impact knocked Peterson unconscious, so he wasn't able to do anything to intervene.
You can see what happens after that in the video after the jump. Peterson, an experienced racer, came to after the medics cut him out of the car and is said to be at home and ok.