Shinobi-iri (忍び入り) is a stealth technique and system of methods for infiltration particularly that studied in Ninjutsu. It was used by ninja agents secretly entering an area such as an enemy camp. Techniques for silent movement and climbing were combined with a range of specialized tools agents could rely on for their mission. Shinobi (忍び) means full-fledged ninja

Entry methods

A system of five methods was used to enter an enemy zone. These Shinobi-iri are:

  • Nyukyo no jutsu - correct timing
  • Monomi no jutsu - finding weakness in defenses
  • Nyudaki no jutsu - finding psychological weakness
  • Yoji-gakure - distraction
  • Joei-on jutsu - erasing light and sound

Nyukyo no jutsu involved the correct use of timing to make entry. Monomi no jutsu is to discover a weak place in the defences. Nyudaki no jutsu is finding and using a weakness in the psychological makeup of the guard, da referring to idleness, and ki means a dislike for being industrious. So essentially it was to find the point where the guard would not be bothered to respond. Yoji-gakure no jutsu involved the classic move of throwing an object in order to distract a guard's attention. Yoji means "toothpick" though commonly a stone or coin was thrown. Toothpick refers to the distraction being minor. When the guard went to investigate the noise the agent would slip past. Joei-on jutsu dealt with obliterating sound and light. It involved such things as keeping light sources between themselves and the guards, moving downwind to avoid alerting dogs and to reduce the likelihood of being heard. For example, by keeping a light between themselves and a guard in order to distract him, the ninja would also make it harder for any guard to see them, due to the difficulty of seeing beyond the light into the dark background where they were. So in Joei-on jutsu, light was being used to obliterate themselves, and sound was erased by positioning themselves where it travelled away from their targets.

Stealth arts

Further, the Stealth skills of the Shinobi[2] or arts of invisibility are these:

  • Inpo the art of concealment
  • Tonpo the art of evasion and escape
  • Shinobi-iri the art of silent movement in darkness
Guidelines for Stealth Walking by Stephen K. Hayes
1. Maintain balance control by allowing your body weight to sink and be carried by deeply flexed knees. 2. Remember to breathe along with your movement. Unconsciously holding your breath can unwittingly produce unnecessary muscle tension, and could result in gasping release of breath if startled or accidentally unbalanced. 3. Stay alert to the entire scene. Do not become so engrossed in watching your feet that you do not notice other people or a new element entering the surroundings.
4. Use all joints for movement, emphasizing fluidity through the engagement of the ankles, knees, and hips for stepping. Avoid the lazy and dangerous habit of stiffening knees and swinging the entire leg from the hip. 5. Maintain your weight and balance on your grounded leg while you move the other leg into position to bear the weight. When absolute silence is a must, avoid distributing your weight over both legs at the same time. 6. If practical, allow your hands to float lightly in front of and beside your torso, one arm higher and one arm lower, to detect possible obstructions before your committed body weight encounters them.
7. Pause and hold your position if you feel that you have accidentally caused too much noise. Listen for signs that you were heard, such as the movement of others or the immediate silencing of background noise following your slip. Sink a little lower on your knees to physically relax muscles that could normally jump with alarm. Take a deep breath and release it slowly to further relax. Continue your pause for as long as you feel is necessary to regain composure and allow possible listeners to decide they did not hear anything after all. 8. Be as patient as possible. If speed of travel is not important, take as much time as you can. Impatience and the resultant hasty movement that it encourages are the greatest dangers to the person who must move silently without detection. 9. Keep your movement appropriate to your surroundings. Do not go to greater lengths than necessary to conceal your movement, while at the same being aware of what others entering the area may see if they cannot hear. Total silence may not be needed when moving through wooded or densely populated areas where scattered noise is a natural part of the environment. Also be aware that low profile crawling or sliding may be the only way to move silently without being seen in some locations.