On Killing by LtCol Dave Grossman, 2009, Excerpts
At hand-to-hand combat range the instinctive resistance to killing becomes strongest. The horror associated pinning a man down and feeling him struggle and watching him bleed to death is barely tolerable.
An obvious method of killing an opponent involves a crushing blow to the throat, yet it is not a natural act; it is a repellant one. The single most effective and mechanically easiest way to inflict significant damage on a human being with one’s hand is to punch a thumb through his eye and on into the brain, subsequently stirring the intruding digit around inside the skull, cocking it off toward the side, and forcefully pulling the eye and other matter out with the thumb.
High-level students trained in this killing technique practice punching their thumbs into oranges held or taped over the eye socket of an opponent. The process is made even more realistic by having the victim scream, twitch, and jerk as the killer punches his thumb to the hilt into the orange and then rips it back out. Few individuals can walk away from their first such rehearsal without being badly shaken and disturbed by the action they have just mimicked. The fact that they are overcoming some form of natural resistance is obvious. Indeed, it is almost too painful to think of it. This procedure of precisely rehearsing and mimicking a killing action is an excellent way of ensuring that the individual is capable of performing the act in combat.