Why Boosters Work

Boosters improve how the vehicle safety belt fits the child and how the child fits the vehicle seat.

The booster raises the child up, making the shoulder belt fit correctly.
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 seat is shallow, eliminating the need to slouch.
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 includes seat belt guides to prevent the seat belt from approaching the child's abdomen.
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."

Boosters in Action: Booster vs. No Booster
Booster seats work in conjunction with lap and shoulder belts to restrict head movement during a crash.  The farther forward the head moves, the more likely it is to be injured.  These diagrams, which are courtesy of the Children's Hospital of Philadelphia, show the difference in head movement in three different scenarios.

Lap and Shoulder Belt worn incorrectly, No Booster
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!

Lap and Shoulder Belt worn correctly WITH Booster
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.