As most dangerous collisions tend to be frontal collisions normally occurring at high speeds, children in a forward facing seat are thrown forcefully forwards until they are restrained by the harness, which in turn prevents them from being ejected from their seat. In a rear facing child seat the deceleration load is spread across the shell of the car seat, so is less concentrated on the head and neck, which in turn minimises head movement and the risk of serious injury. Click this link for full article on forward facing vs rear facing child seats.