How does Newton's first law explain how seat belts prevent injuries in car accidents?

How does Newton's first law explain how seat belts prevent injuries in car accidents?
An object at rest tends to stay at rest and an object in constantmotion tends to stay in motion unless acted upon by an unbalancedforce. The first law applies in this case. The car and its contentsare moving at a constant rate and everything is fine. When suddenlythe brake is applied, since the human is not tied to the seat withnuts and bolts, it tends to keep moving forward at the same speedthat the car was travelling before the brake was applied. Since itis rather unwise to tie the human to the seat with nuts and bolts,the next best thing would be to provide a restraining agent tominimize the forward motion of the body and yet give him freedom ofmovement to operate the vehicle. The buttocks of the driver are abit more rigidly fixed to the seat (relatively speaking) than theupper torso. The whole body continue to move at the speed at whichthe car was travelling before the brake was applied, but the uppertorso moves forward farther and faster. The shoulder belt tries tominimize this speed lest the driver hit the steering column. Thelap belt provides the same function to the lower portion of thebody.