The shoulder belt crosses the center of the chest and rests between the shoulder and neck. The child is comfortable – and therefore not tempted to place the shoulder belt behind the back or under the arm (both of which are dangerous)
The booster’s shallower seat allows the child’s knees to bend comfortably over the edge without having to slouch. The child’s back rests against the back of the vehicle seat (in a backless booster) or booster seat (in a high-backed booster), allowing the safety belt to tighten appropriately against the child’s upper body. The child is again more comfortable and of course safer.
The booster has either small handles, arm rests, guides or slots that help position the lap portion of the belt low and flat across a child’s upper thighs and prevent the lap belt from riding up onto the abdomen. This eliminates the risk of “seat belt syndrome.”
In the first diagram, a child who should be using a booster seat is instead using just a lap-and-shoulder belt. However, he has put the shoulder portion of his seat belt behind his back, which means his upper body is not restrained. Look how far his head moves in a crash!
In the second diagram, a child who should be using a booster is instead using just a lap-and-shoulder belt, albeit correctly. His head still moves quite far forward during a crash.
In the third diagram, the child uses a booster correctly. His head barely moves forward and he will sustain much less serious injuries than the other two children in the above diagrams.