How does a seat belt affect your momentum during an accident?

Contrary to popular belief, seat belts are not a cure-all for preventing injuries in car accidents. if you hit something, when the car decelerates, while your outside body is part of the car due to the seat belt, your insides still move forward at the speed the car was traveling. if you crash while going fast enough and the car decelerates fast enough, your insides will crash into the inside of your body (your liver collides with the inside of your abdominal wall, your heart/lungs collides with the inside of your rib cage, your brain collides with the inside of your skull, etc) just like your whole body would crash into the dashboard without your seat belt on, causing internal injury, even if the outside of your body doesn't come in contact with the interior of the car in any way.