The ninjas were able to impress those people of the old times as they one foot ahead of them technologically.  By “technologically” I mean for the weapons and gadgets but they were not yet at the age of electronics or anything of this sort!  They were just able to make ingenious weapons out of their farming tools or everyday utensils.  Indeed ninjas relied heavily on their technological advancement over the samurais to win since they were weaker physically than the latter (after all, the ninjas were originally farmers!).

            Through their exile in the mountains, they learned that to survive out there, they had to understand their surroundings, the nature and be closer to it.  This way, they became somewhat “masters of the forest”.

In fact it is the Ninja's understanding of invisibility that gives them the advantage. Invisibility, in reality, is simply knowing how to fool the enemies’ senses in order to remain undetected, much like a magician who misdirects his audience to focus on other things in order to mystify and confuse.  There are two natural senses that are the focus of study.

The art of invisibility (or the art of hiding) - Inpo

            Many people thought that the ninjas could become invisible, as the latter often broke into castles unseen.  In fact, there is no such thing as becoming invisible, literally talking!  It is all based upon the Internal Energy (Ki) of a person, the knowledge of moving in the dark, knowing how to fool enemies and of course patience (nin).  Inpo gave rise to the legends that the ancient Ninja could vanish at will.  The internal energy is represented as a flame glowing at the center of your chest and it represents your “presence”.  Obviously one cannot just extinguish this flame (or this mean he is dead!), but one can lower its intensity.  This is what ninjas do to prevent detection; they lower their presence.

Some interesting stuff:

If you do not understanding what the “ki” is and how it represents a “presence”, read this!

Next among the precepts of Inpo is the admonition to avoid unnecessary movement.  Indeed ninjas always come to the place where they had to operate a day before to gather intelligence and reconnoiter the area for possible hiding places, and escape points.  This way, they would know exactly where they had to go, thus avoiding unnecessary hassle during the actual mission.  And certainly, they came disguised.

The two natural senses that the ninjas suppressed in order to remain “invisible” are sound and sight.

There are 2 arts that study sound perception and control; the “Himitsu Iri”, meaning the ever-elusive method of “Silent Movement” and the “Kage aruki”, meaning “Shadow Steps”.

The “Himitsu Iri” teaches the ninja to approach anything in a subtle manner and of course remain as secretive as possible.  This is based on the fact that noises usually attract attention and if the mind fails to identify the sound, it becomes curious and may want to investigate further.  With this art, the ninja minimizes as much as possible these sounds.

To remain invisible at all times, the ninja could not afford to walk normally and developed several patterns of walking silently called the “Kage aruki”.  This art conditioned a ninja to be able to walk or even run on any kind of terrain (foliage, rooftops, squeaky floors, etc) while making the least sound possible.  This of course cannot be achieved in a year or so of training but demands a lifetime training to become very proficient.

 Some say that seeing is believing.  Indeed the ninja used this saying as the basis of another art in the field of invisibility.  For the ninja, if something is visually perceived, the value of surprise is spoiled.  Therefore, it is important that the ninja developed certain techniques to stay out of sight of enemies who are in their visual range.  To do this, the ninja had four “tools” namely the Direct vision. the Petrified vision, the Blocked vision and the Mind’s eye.

Some important arts of stealth

KAGE ARUKIJUTSU - THE ART OF SILENT WALKING  

It was the ninja field agents perfected the “Kage Aruki” walking techniques during the course of their missions. It is the use of proper weight distribution, mental awareness and general feats of dexterity that enable an operative to carry out these acts of stealth.  To move soundlessly requires that the ninja commit him to countless hours of practicing and perfecting their technique. They traversed every type of imaginable terrain so that when specific surfaces are encountered during missions, they are fully prepared to move through or on anything silently. However, these walking techniques alone do not magically endow one with the ability to move unheard. These steps are simply tools used to increase ones ability to move with more grace and elegance. In actuality, stealth becomes a skill the warrior attains without really being aware of it. It is also a result from their unarmed, acrobatic and weaponry training. The ninja will only be successful in applying these techniques when he has attained an intimate knowledge of his body and how it works.

HAYAGAKEJUTSU - RUNNING SKILLS  

The art of "Hayagakejutsu" is one that is used to develop greater speed and stamina when running. These techniques require that the Ninja go through a series of rigorous training that are designed to increase speed by applying different principles of body weight distribution. These running techniques are of vital importance since they efficiently combine both stealth and speed.

When using “Hayagakejutsu” the Ninja assumes a low Hiryu posture with the upper body leaning deeply forward. As the warrior runs in this position, he is not only using the power of his legs, but also his upper body because it actually pulls the body forward enabling him to run faster. The body must not bob up and down as the Ninja runs and the arms should not move at all. Instead, the arms should remain as motionless as possible so as not to attract unwanted attention. The legs are the only part of the body that alternates the  motion and direction.

HOFUKUJUTSU - CRAWLING SKILLS  

Another unique method of silent movement is the art of "Hofukujutsu" in which the Ninja employs different methods of low movement to avoid detection.  One such method is the "Ryohebi no jutsu," or the "Serpent crawl".  Using this technique the ninja walks on all fours with his stomach very low to the ground. The serpent crawl provides the Ninja with a very fast method of low movement when traveling beneath the enemy's line of sight. One of the best crawling techniques is that of the "Shinso toho no jutsu," or the "Deep grass rabbit" technique. This tactic is used when the Ninja is moving in extreme darkness and wants to avoid making noise by tripping or bumping into anything. This technique is accomplished by walking on all fours while scanning softly in all directions with the hands and feet. Although this is a very slow process, it is also very effective. 

GIONJUTSU - FALSE SOUNDS  

Even good ninjas sometimes make mistakes and walk on some dried branches that will alert any enemies nearby of their position.  Being aware of their possible human errors, they have developed techniques to cover or muffle those unwanted sounds.  This is accomplished through the use of a very ingenious and extremely effective art called "Gionjutsu", the art of employing "false sounds". Using this art, the Ninja creates noises that will deter attention from his presence. Therefore, if an enemy sentry was patrolling the land and he hears a strange noise, he would most certainly investigate. The Ninja can then produce false sounds because often a simple sound will calm the sentry's suspicion.

The sounds that the ninja could make range from the sound of a cricket to that of a cat or dog.  In fact the sound of the cricket is most efficient since crickets usually stop chirping when they are approached.  On hearing this sound the sentry may not investigate any further since he may think that if there were someone there, the cricket would cease his song.  The sound of a cat is also very effective since no sentry will want to make a fool of himself by investigating a cat (they were supposed not to fear such small animals, that’s why!). 

Even though this tactic is creative, there is a chance that the enemy will not fall for the ruse.  Then, physical combat may be the only solution. 

DOBUTSUJUTSU - THE ART OF USING ANIMALS  

Another useful tactic that you may not classify as falling into the stealth and invisibility section is the "Dobutsujutsu," or the art of "Using animals”.  This tactic makes use of live animals as a means of diversion.

To remain “invisible”, the Ninja may release a small animal such as a dog, cat, rabbit or monkey into the area of operation and create diversion thus preventing his discovery. This technique is used when the ninja feel that the threat of being discovered is imminent.  These animals often cause havoc with the enemies’ senses while the Ninja slips away quietly.

This tactic can also be used with insects such as venomous hornets or bees.  In which case, the ninja carry a hive of angry insects in a pouch.  When he detects the presence of enemy forces he shakes the bag to enrage the insects and then throws the open pouch in the direction of his enemy. By the time the bag lands, the insects will be so furious that they will attack anyone who are close by. 

 KAGEJUTSU - INVISIBILITY  

To truly embrace the concept of invisibility the Ninja must not only be undetectable to the ear, they must also remain equally unnoticed by the eye.  Remember what I said earlier “Seeing is believing” to the normal person! The human scope of visual perception is the focal point in this situation. Visual perception is a fairly simple concept. The ninja must come to learn how different types of vision affect their chances of remaining unseen.  This art also favors the use of shadows and shapes all around the ninja.  For example, the use of foliage in a forested area may fool the eye of an observer.

KAGESHIN - THE QUIET MIND  

Stealth is largely a mental activity. A ninja must learn to control his mind and thoughts and may achieve this through the art of "Kageshin," or "Quiet mind".  This art teaches the ninja to free his mind of aggressive intentions and be at peace.  In other words, the ninja’s mind must become “silent”.  Because the body usually tries to imitate or follow what the mind is thinking it is a particularly important task to know how to suppress these thoughts.  For example, thoughts of anger or anxiety may lead to errors in judgment in a situation.  What this means is that the ninja's mind must be alert to what is happening around them without any anxiety of being detected. The ninja must simply be confident in their training, knowing they have been prepared to deal with every eventuality.

The art of disguise

            Ninjas were also masters of disguise for it was an essential art for traveling around the country unsuspicious.  But this art also required that the ninja had some talent and skill at certain disguise that they would use.  For example, they could disguise them as entertainers, but to prove that they really are entertainers, they should perhaps be able to juggle or do a comic scene.  If they decided to be a monk, some religious knowledge was required.   Ninjas are especially skilled in five general categories of disguises. These disguises are called the "five ways of going".

Ninja and Magic

Ninjas tend to disregard magic and rely on their technological devices and finely honed skills. When they do resort to magic, spirit magic is most commonly used since ninja, living close to the land, find it easy to interact with the nature.  Some ninja clan did practice some kind of sorcery. Indeed, a few clans specialize in sorcery rather than using devices. Such clans favor castings that duplicate the effects of other clans' devices (if ever that could work!).