Correct me if I am wrong, but I from what I remember is that they measure the G-forces on the helmet/head/neck when dealing with helmet impacts. Thick pads are placed on the inside, to soften the G-force, as well as spread it out over the entire head instead of a small area. Combined with a suspension system, a lethal impact g-force wise, can be reduced to a survivable level. And like mentioned before, chances are the shot won't be from point blank which will also reduce the impact force.