How does inertia effect a person who is not wearing a seatbelt during a collision

The car is moving forward and according to human perception, so is the human body. During the collision the car suddenly changes its direction and stops the movement, but the human body by inertia still keeps moving forward and will cause the person to hit whatever is in front of it with a great force, consequently causing injury. The seat belt is sensitive to sudden movements and in the time of collision stays firmly in place - it does not expand, and stops the person from moving forward. A person who is not wearing a seat belt has nothing to control inertia.