If a seat belt is improperly worn, it won't do what it's designed to do in protecting your body in the event of a collision. If you don't have the torso strap over your chest or torso, it is not uncommon for you to be able to slide out from underneath of the seatbelt. For people who are smaller in stature, such as toddlers and youth, they don't just wear the seatbelts; they have to wear the seatbelts in addition to a car safety seat or a booster seat, to put them in a place where the seatbelt will be able to do its job.
On April 2nd, 2019. See this statement in context.