Spiritual Power (also known as "Reiryoku'") is a power aligned with the spiritual sensitivity and willpower of the user. It is used by Soul Reapers to provide power for their various feats and to effectively channel the powers of their Zanpakuto. It can also be used to cast Kido, magic spells created by the Soul Reapers. Reiryoku 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 Reiryoku into the air. The aforementioned ability is known as Spiritual Pressure (also known as "Reiatsu"), possessing power that could be literally felt within the atmosphere.
Every spiritual being and every human has a certain amount of Spiritual Power . 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 Reiryoku have is the ability to see ghosts: Pluses, Soul Reapers and Hollows alike. Soul Reapers are essentially departed souls in the Soul Society with very high levels of Spiritual Power who receive special training to utilize their powers. Hollows, if they make active use of their Spiritual Power, usually do it to catch their prey or to evade Soul Reapers more effectively, the extreme case being the Arrancar, who have an amount of Spiritual Power equal to Soul Reapers, and are able to use it just as efficiently in combat.
Spiritual Power Levels
This level of spiritual power corresponds to those possessing an average level of spiritual power. This level of power is common among those of significant spiritual awareness and may even possess powers of their own but the power levels can vary among individuals. This is also a common level of power for a low ranking seated and non seated members of the Thriteen Court Guard Squads or lesser power individuals.
High Spiritual Energy
This level of spiritual power corresponds to those possessing an above average amount of spiritual power, which is finely controlled. This level of spiritual power is common amongst those of an average lieutenant rank and high ranked seated officers.
Great Spiritual Energy
This level of spiritual power corresponds to those possessing a large amount of spiritual power which is finely controlled. This level of spiritual power is common amongst those of an average Captain rank.
Immense Spiritual Energy
This level of spiritual power corresponds to those possessing an enormous amount of spiritual power which is finely controlled. This level of spiritual power is reflected in its ability to enhance a user during combat as well as its ability to affect the surrounding environment on a controlled level, allowing the wielder to use it in combat to startling degrees.
Vast Spiritual Energy
This level of spiritual power corresponds to those possessing an enormous amount of spiritual energy which is wild and untamed. This level of spiritual power has a possible tendency to leak from a person or when released can affect the surrounding area in a destructive way. Those of this level have no actual masterful control over their massive spiritual power, as it can effect those around them unintentionally.
Sensing Spiritual Power
Another basic ability that beings with higher-than-average levels of Spiritual Power possess is the ability to sense 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 Spiritual Power, which can be sensed. This ability apparently requires some innate skill, but can be increased with training. Initially, Ichigo Kurosaki's ability to sense the spiritual energy of others was very poor (mainly because his own abundant supply drowns out that of others preventing it), but gradually grew much more proficient in it, even being able to use various high-level techniques to locate or detect particular people, such as his close friends (most notably Yasutora Sado).
There is another way to sense another spiritual being's Spiritual Power, if one undergoes the process training in the arts of Senjutsu. As a Sage, one can channel alot of natural energy, once activation of Sage Mode, they can sense his/her's Spiritual Power.
Every spiritual being has a unique pattern to his/her emitted Spiritual Power. 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 Spiritual Power pattern can mean that the person in question is in great stress (i.e. fighting) or has received a grave wound.
Different types of spiritual beings have greatly different patterns, the Spiritual Power pattern of hollows for example is easily distinguishable from that of Wholes or a Soul Reaper. The Visored are a special case, because when their Hollow mask is donned, their pattern of Spiritual Power resembles that of a Hollow, rather than that of a Soul Reaper.
There are a few possibilities to block Spiritual Power. One is the usage of Sekkiseki, a special kind of stone that completely blocks Spiritual Power from both sides. The walls of Seireitei, and the walls of the Tower of Punishment are built from this stone. As Lethality Stone blocks even the strongest Spiritual Power emissions, it does not just inhibit the sensing of one's Spiritual Power 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.
Another means of blocking Spiritual Power is a special garment invented by Kisuke Urahara, which completely hides any emission of Spiriutal Power coming from the wearer, enabling him/her to escape from somewhere or approach someone while being impossible to notice via sheer sensing of Spiritual Power.