The Spring Brake Thingy is designed to protect your expensive transmission from damage when parking on a hill and having that "BANG" when trying to re-engage into gear. It is not designed to be used as sole brake system while parked on inclines, always use extra precaution and use large blocks or rocks behind your tires to prevent rolling.
Your brakes must be in good working order, this product will not compensate for brakes not properly working. Do not stand in front of or behind the vehicle when on an incline, always keep front and back area of vehicle clear, even with parking brake engaged. There is no full proof system on any vehicle that will guarantee a vehicle from rolling while on an incline.