Bleach extreme
Spiritual Energy or Reiatsu is a power aligned with the spiritual sensitivity and willpower of the user. It is used by Soul Reapers, Awakened humans, Quincy’s and Hollows to provide power for their various feats, powers and to effectively channel the powers of their Zanpakoutu. 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.
Every spiritual being and every human has a certain amount of Spiritual Energy. If this amount is higher than a certain degree, it grants the person superhuman abilities. Those with such an above-average Spiritual Energy are very rare among living humans. It seems to be that the most basic ability that humans with a higher level of Spiritual Energy is that of being capable of seeing ghosts, Hollows, Soul Reapers and Hollows alike. Soul Reapers are essentially departed souls in Soul Society with very high levels of Spiritual Energy who receive special training to utilize their powers. Hollows, if they make active use of their spiritual power, usually do it to catch their pray or to evade Soul Reapers more effectively
Another basic ability that beings with higher-than-average levels of Spiritual Energy 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 Spiritual Energy, which can be sensed. 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 Spiritual Energy pattern of Hollows for example is easily distinguishable from that of Wholes or a Soul Reaper
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 awaked humans are capable of performing many superhuman feats with the help of their Spiritual Energy. 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, Soul Reapers and other dwellers of Soul Society need food to replenish their strength, while on the other hand, when doing everyday activities, they do not need to eat anything to sustain them.
Aside from Spiritual Pressure, Shinigami generally use their Spiritual Energy to
cast spells - see Kido
increase their speed and their height of jump, or even fly, walk and float in the air - see Hoho
use the special abilities of their Zanpakuto
Arrancar and Hollow have other spell-like abilities, Cero and Bala, these are basically condensed Spiritual Energy 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. Arrancar and Hollow can also increase their mobility like the Shinigami above, and the usage of their Zanpakutō also relies on the usage of Spiritual Energy, albeit in a different manner.