Unfortunately, the Motocross neck braces can not guarantee a 100% accident prevention, but they can certainly be very useful to reduce injuries due to accidents.
Obviously the main function is to protect and support the neck and the cervical area, but there are different models that perform this function in a slightly different way. Some limit themselves, in fact, to protect from damage due to an impact, also limiting the unnatural movements that the neck could have during an accident. More advanced types are a step forward, offering support and protection to the chest and shoulders. Neck support is essential for limiting helmet movements in the event of an accident, preventing hyperflexion and hyperextension of the cervical muscles. In the event that the motorcyclist suffers a head impact, the helmet carries the force of the collision to the cervical support, which directs it to the shoulders and chest, saving the neck from a pressure that it could not handle. A little 'as it happens with the other elements of your equipment, even for cervical support is impossible to protect yourself from any kind of impact (given the infinite possibility of angles and dynamics that may have a collision). It follows that, even just to significantly lower the chances of a serious injury, wearing a neck brace can only be considered a sensible idea!