The hydraulic pressure that is believed to effect lights out kills isn't a 100% sure thing. Some of the buffalo in the study Jim Carmichael referenced died instantly with broadside rib shots. Some stayed on their feet until loss of blood and loss of blood pressure caused them to collapse a bit later. Autopsies revealed that the brains of buffalo that went down instantly at the time of bullet impact experienced massive bleeding within the brain (ruptured veins/arteries). No such brain bleeding was observed in the buffalo that remained on all fours for some period of time following bullet impact. This is what caused the study to end with the explanation/conclusion that if the hydraulic pressure of the bullet impact is added to maximum heartbeat blood pressure in the animal, veins and arteries are more apt to burst than if the the bullets' added pressure wave occurs at low heartbeat blood pressure.
So we'd like the bullet to impact at the time when the animals' blood pressure is peaking for maximum lethality. During each heartbeat, blood pressure varies between a maximum (systolic) and a minimum (diastolic) pressure. Maybe if we break the trigger at the same time a hunting buddy hollers - HEY YOU!!!
The impressive killing effect caused by high speed bullet impacts has been documented. It has not been documented to happen 100% of the time, with each and every shot. But once you've observed it first hand, you'll be left shaking your head in amazement.