Reiatsu

Reiatsu, or Spiritual Power, is a power aligned with the spiritual sensitivity and willpower of the user. It is used by Shinigami to provide power for their various feats and to effectively channel the powers of their Zanpakutō. It can also be used to cast Kidō, magic spells created by the Shinigami. Reiatsu can even be coalesced into a physical form and thrown or shot as a projectile. As a display of potency, spiritual beings have the ability to incapacitate the opponent's movement and simultaneously frighten them, by means of dispersing Reiatsu into the air. The aforementioned ability is known as Spiritual Pressure, possessing power that could be literally felt within the atmosphere.

Overview
Every spiritual being and every human has a certain amount of Reiatsu. If this amount is higher than a certain degree, it grants the person superhuman abilities. Those with such an above-average level of Spiritual Energy are very rare among living humans. It seems to be that the most basic ability that humans with a higher level of Reiatsu have is the ability to see ghosts: Pluses, Shinigami and Hollow alike. Shinigami are essentially departed souls in Soul Society with very high levels of Reiatsu who receive special training to utilize their powers. Hollows, if they make active use of their Reiatsu, usually do it to catch their prey or to evade Shinigami more effectively, the extreme case being the Arrancar, who have an amount of Reiatsu equal to Shinigami, and are able to use it just as efficiently in combat.

Sensing Reiatsu
Another basic ability that beings with higher-than-average levels of Reiatsu have that they are capable of sensing other spiritual beings without relying on their five basic senses. This is possible because every spiritual being emits a certain amount of his/her own Reiatsu, which can be sensed. This ability apparently requires some innate skill, but can be increased with training. Ichigo Kurosaki can sense the spiritual energy of others very poorly, but he gradually becomes more adept in this skill.

Every spiritual being has a unique pattern to his/her emitted Reiatsu. One can recognize the pattern of his friends and acquaintances after some time, being able to sense their presence and their physical and mental state from relatively great distances. Sudden changes in Reiatsu pattern can mean that the person in question is in great stress (e. g. fighting) or has received a grave wound.

Different types of spiritual beings have greatly different patterns, the Reiatsu pattern of hollows for example is easily distinguishable from that of Pluses or a Shinigami. The Vizard are a special case, because when their hollow mask is donned, their pattern of Reiatsu rather resembles to that of a Hollow, in contrast to that of a Shinigami.

There are a few possibilities to block Reiatsu. One is the usage of Death Stone, a special kind of stone that completely blocks Reiatsu from both sides. The walls of Seireitei, and the walls of the Tower of Punishment are built from this stone. As Death Stone blocks even the strongest Reiatsu emissions, it does not just inhibit the sensing of one's Reiatsu pattern, but effectively closes out most unwanted visitors in the case of Seireitei, and prevents the escape of even the strongest prisoner from the mentioned tower.

Controlling Reiatsu
The aforementioned emission of Spiritual Energy is a common and natural phenomenon, but a spiritual being can also control the degree of this emission, this usually requires a certain amount of skill and training. By completely blocking his/her Reiatsu "pouring out", one can completely hide his/her presence from other spiritual beings and spiritually aware humans. On the other hand, amplifying this emission is known as Spiritual Pressure, and is considered a show of force, or even an attack in itself, and it can indeed paralyze opponents, and cause direct damage for example in the built environment.

Aside from controlling the amount of emitted Reiatsu, most spiritual beings and some of Ichigo's spiritually-aware human friends are capable of performing many superhuman feats with the help of their Reiatsu. These feats usually require a certain level of concentration, and they tend to tire the person after some time, or after performing some of them in quick succession. It is also known, that after excessive usage of Reiatsu, Shinigami and other dwellers of Soul Society need food to replenish their strength, while on the other hand, they do not need to eat anything to sustain themselves during everyday activities.


 * Aside from Spiritual Pressure, Shinigami generally use their Reiatsu to
 * cast spells - see Kidō
 * increase their speed and their height of jump, or even fly, walk and float in the air - see Hohō
 * use the special abilities of their Zanpakutō
 * increase their strength and power
 * Hollows and Arrancar have other spell-like abilities, Cero and Bala, these are basically condensed Reiatsu fired towards the opponent. In the case of the former it is done in a flame-thrower-like manner, in the latter case, the fired Reiatsu is bullet-like. Hollows and Arrancar can also increase their mobility like the Shinigami above, and the usage of their Zanpakutō also relies on the usage of Reiatsu, albeit in a different manner.


 * Quincies can also increase their mobility, and form a weapon (usually a bow) from Spiritual Energy. In their usage of Reiatsu however, there is a fundamental difference from spiritual beings - they primarily rely on collecting Spiritrons from the environment, and use this collected Spiritual Energy to fight. They can even collect and store Spiritrons and therefore Spiritual Energy in various special items.