All ninjutsu practitioners should understand that they must be aware of their surroundings and that includes, perceiving a potential attacker. All people give off valuable clues as to their current state of aggression or potential actions. Let’s start off with what we see first someone’s Face.
The face is on one hand the most expressive body part we possess, and on the other hand, the most easily manipulated. Experts are able to meticulously control their facial expressions so as to be unreadable (eg. the poker face) while amateurs will crack nervous grins and will sport numerous facial twitches. Ignore the signs that can be controlled and thus manipulated, and focus on those that cannot:
Pupil dilation: The human fight or flight reaction is something few people can control; the brain signals the body to dump adrenaline into the bloodstream raising the heart rate and dilating the pupils (making them larger). As the pupils dilate, the peripheral vision narrows – it is a mechanism designed to have us face the threat directly. People about to act aggressively or perform a violent act will usually have their pupils dilated the size of pie plates.
Pulse: As aggression or impulse builds, the heart rate increases as well as blood pressure. The net result of this is a pounding pulse which is visible in the neck and temples. Again, this is difficult or impossible for most people to control.
Sweat: An increased heart rate causes involuntary perspiration, which again, not even professionals can adequately control.
Mouth: Besides obvious expressions, an open mouth often occurs when a person can’t get enough air from just their nose and is breathing rapidly.
The ability to recognize these early warning signals could give you valuable seconds in which to act, potentially saving your or a loved one’s life. So learn and practice these lessons well.