I think it's also something to do with agression and speed. Either you slow down when low on health, therefore giving you a better chance, or you speed up, making you harder to hit and harder to manuver against. For example, in Modern Warfare 2's Co-op Special Ops mode, with just seconds on the clock, me and my friend were forced to sprint the final stretch, just spraying the enimies as we went. Despite the veteran difficulty, we made it... just about,because we just ran past most of the bad guys, rarther than sticking about to shoot them and be shot.