The term “stop” is loosely defined as the threat stops all hostile action. A stop might just as easily be a look or body posture as multiple bullets tearing through vitals. There is a tendency in firearm circles to confuse a ’stop’ with a ‘kill’, but this is incorrect. Handgun bullets do not tend to kill people stone-dead on impact, though they might deter future hostilities.
There is a lot of discussion about the ’stopping power’ of some given caliber, cartridge and bullet configuration. For the most part these discussions are irrelevant, as all major caliber handguns seem to produce the same range of effects. Handgun bullets produce low velocity wounding, so a few more millimeters of wound channel or another inch of penetration is not really all that significant. The ballistics might differ, but the physiological effects are all pretty much the same.
We desire to effect “stops” against all assailants, but are responses have to be appropriate for the circumstances. The assailant that runs away because of a face full of pepper spray is stopped, but he is not killed (or even harmed.) For self-defense purposes we would like to be able to effect stops with the lowest level of force possible, but we must recognize that anything less than incapacitation is a deterrent, and requires the assailant to want to quit. All stops occur via one of three mechanisms: psychological, physiological, and neurological. Or as Greg Hamilton, once said “People stop because they run out of blood, brains, or balls.”
A psychological stop is where the assailant quits. Pain, fear, or uncertainty cause the assailant to choose to break off the victimization. All applications of less than lethal force (and a good number of lethal applications as well) can be said to be psychological stops.
A physiological stop is where there is enough damage to the assailant that he is unable to continue. Large amounts of blood loss that leads to unconsciousness, or breaking essential pieces of his skeletal structure that are necessary to keep him engaged in the fight are examples of physiological stops. It is important to realize that damaging non-vital organs might cause death given enough time, but they are not fight-stopping injuries, and are not physically incapacitating.
A neurological stop is simply damaging the brain so that it can no longer direct the bodies functions. This is the most reliable stopping mechanism, but it is the hardest to effect. The brain is well protected and most the tools available in self-defense situations have a hard time penetrating the skull.