Kindly explain how a person will come in contact with the airbag (with the possible exception of the head) if the shoulder harness immediately locks, in the event of a suitiable deceleration force, and prevents the upper body from moving forward?

I do agree that the airbag, if it deploys properly, may stop the forward motion of the head, thereby preventing a neck injury, however, I fail to see how the bag can prevent breaking of a clavicle, if the majority of the restraint is provided by the shoulder harness.