Hydrostatic Shock is a key component in the effectiveness of modern day firearms. Two facets of Hydrostatic Shock that readers of Max Brooks will be familiar with are the Balloon Effect and Sudden Nerve Trauma.
The only way to immediately incapacitate a person or animal is to damage or disrupt their central nervous system (CNS) to the point of paralysis, unconsciousness, or death. Bullets can achieve this directly (through physical penetration of the brain or spinal cord) or indirectly. One indirect method is damaging the cardiovascular system so that it can no longer provide enough oxygen to the brain to sustain consciousness, and celluar function. Unless a bullet directly damages or disrupts the central nervous system, a person or animal will not be instantly and completely incapacitated by physiological damage.
However, several scientific papers reveal ballistic pressure (or force) wave effects on wounding and incapacitation, including central nervous system injuries from hits to the thorax and extremeties. These papers document remote wounding effects for both rifle and pistol levels of energy transfer. These effects summarize (to different extents) what Max Brooks was alluding to in World War Z (in describing the ineffeciency of conventional weaponry at the Battle of Yonkers.
Recent peer reviewed research provides compelling support for the role of a ballistic pressure wave in creating remote neural effects leading to incapacitation and injury. In one such study, researchers implanted high-speed pressure transducers into the brain of pigs and demonstrated that a significant pressure wave reaches the brain of pigs shot in the thigh. These scientists observed neural damage in the brain caused by the distant effects of the ballistic pressure wave originating in the thigh. The results of this study were confirmed and expanded upon by a later experiment in dogs. This effect is largely nullified in a Brooks Zombie because their entire body chemistry has been altered into a less fluid state. This gelatainous composition insulates and directs force waves away from the brain, outward. This is why rounds to the body are generally ineffective against zombies, though in reality, this would change little regarding this particular facet of ballistics.
Emotional shock, terror, or surprise can cause a person to faint, surrender, or flee when shot or shot at. Emotional fainting is the likely reason for most "one-shot stops", and not an intrinsic effectiveness quality of any firearm or bullet; there are many documented instances where suspects have instantly dropped unconscious when the bullet only hit an extremity, or even completely missed. Additionally, the muzzle blast and flash from many firearms are substantial and can cause disorientation, dazzling, and stunning effects. Flashbangs (stun grenades) and other less-lethal "distraction devices" rely exclusively on these effects.
Pain is another psychological factor, and can be enough to dissuade a person (but obviously not a zombie) from continuing their actions.
Temporary cavitation can emphasize the impact of a bullet, since the resulting tissue compression is identical to simple blunt force trauma. It's easier for someone to feel when they have been shot if there is considerable temporary cavitation, and this can contribute to either psychological factor of incapacitation. In a high caliber weapon, the temporary cavity itself can cause serious damage as the flesh expands too quickly and violently to rebound, and simply tears apart. This is particularly notable in powerful hunting and sniper rifle rounds such as the .50 BMG, which can blow open an abdominal cavity and sever limbs with a single shot.
However, if a person is sufficiently enraged, determined, or intoxicated they can simply shrug off the psychological effects of being shot. Therefore, such effects are not as reliable as physiological effects at stopping people. Animals will not faint or surrender if injured, though they may become frightened by the loud noise and pain of being shot, so psychological mechanisms are generally less effective against non-humans. This is why fast zombies are more resilient than humans.
As zombies largely renounce their conscious mind, they are entirely immune from the psychological realm, including (but not limited to) all of the effects listed in this section.