FWIW. I read the following on another site. I think the last sentence sums it up pretty well.
When larger animals drop when shot, the best explanation for this occurring is that they have suffered damage to the upper central nervous system. This can be from a high spine shot that will usually result in instant death, and if not, then immobilization (paralysis) such that the animal may be dispatched quickly. Instant drops can also be from a shot to the brain stem or the brain (instant unconsciousness and death).
Aside from these direct impacts, the upper CNS may be damage by a near miss and the nerves damaged by hydraulic shock. For example, you attempt a neck shot and the bullet passes just beneath the spine, but the temporary wound cavity (caused by hydraulic shock) envelopes the spine and causes tearing or other damage to the nerves.
Anther indirect method is hydrostatic shock. This is the damage of the brain stem or brain by a shot elsewhere in the body (say, the shoulder) where the energy pulse of the impact travels via the vascular system (primarily) and slams into the fairly delicate nerve centers of the brain. I have seen hogs die, for example, but a shot that simply passed through muscle and soft tissue of the shoulder without taking out the heart, lungs, spine, or major blood vessels, yet the hog went down on the spot. The only realistic explanation is hydrostatic shock.
Hydraulic shock and hydrostatic shock cannot reliably be counted on to drop/kill and animal. There is some relationship between the size of the animal and the velocity and size of the bullet impacting the animal and whether or not these may occur. Whereas a mouse may be overcome by a .22 lr shot, a similar shot with a .22 on a bison may largely just make the bison mad. There isn’t enough of an impact on the bison from the tiny and slow bullet as compared to what the same bullet would do to a mouse. Make sense?
Again, these indirect methods are not 100% by any stretch. When they do occur, it is a benefit to the hunter, though the hunter doesn’t know when they will occur. Other factors such as the direction of the impact, health of the animal, etc. etc. may influence these as well. Put another way, you could shoot 2 nearly identical deer in the same place with the same caliber and bullet from the same distance (assuming you aren’t shooting the spine or brain) and one deer drop in place and the other run a ways before collapsing.
