The car strap is designed to work in conjunction with your harness to control your dog's movement within your car. While it cannot guarantee your dog will not be hurt in an accident, it will minimize the chance of them becoming a moving projectile or escaping if the car doors come open during an accident.
To avoid injury to your dog’s neck, do not attach to your dog’s collar -- only use this with a harness with a place to connect it to the back of the harness.