Resonance

Resonance (反響 (レゾナンス), Hankyō; Japanese for "Resonate/Influence") is a spiritual phenomenon that can both be invoked and occurs spontaneously between multiple entities in order to produce certain effects. Though it is ideally possible for all beings to induce resonance, it is most commonly seen within those who are not limited by the physical laws of.

Zankensoki Mastery
Among the innumerable applications of the resonance concept, none more so stand out than the mastery of the Zankensoki; the war crafts of the souls. Reaching mastery in any of these forms of combat demands that the practitioner learn some degree of resonance of a particular kind. It is in this sense that the Zankensoki and the physical combat arts of humanity show their vast differences.

seems to rely upon "concept resonance" (概念反響, Gainen Hankyō). In the martial arts, each style and technique finds its origins from studying a certain idea in the world. By watching how the water flows in rivers, movement possibilities arose in the martial arts. The mountains inspired stance, the animals inspired action and reaction, and so on and so forth. A Hakuda Master may resonate with these concepts in order to produce a certain effect that goes beyond causality. They may produce techniques that simply cannot be explained with "cause and effect".

relies upon "soul resonance" (真髄反響, Shinzui Hankyō). When a Shinigami and their Zanpakutō establish a dialect, they enter into a union between souls. The two souls essentially resonate with one another and allow an even greater power to take form, resulting in the Shikai and Bankai forms. This can only take place between a sword and its master that have come to accept one another on some kind of fundamental level, which causes their souls to somewhat spark a connection and results in a resonance. However, the powers that are generated from this union matter greatly upon the two souls in question. There exists a similar situation in regards to Hollowfication.

relies upon "spatial/dimensional resonance" (次元反響, Jigen Hankyō)

essentially relies upon "desire resonance" (欲求反響, Yokkyū Hankyō)

Multi-Soul Resonance
Similar to the soul resonance used by a Shinigami and their Zanpakutō, there exists a multi-soul resonance that allows for a team of souls to essentially form a spiritual link between them, causing them all to become heightened in power, strength, speed, and other associated aspects by a respective amount. An actual river of spiritual power flows between the lot of them in a true demonstration of natural resonance. Additionally, their mind and instincts are also linked together, making it possible to coordinate team assaults to an almost perfect and flawless standard. It makes it so those using the multi-soul resonance can even combine their attacks down to the reishi level, creating amazing new techniques that carries much more effect that either of the individual techniques alone. With such an ability, it becomes possible for a group of seemingly weak Shinigami to fend off a single powerful Captain-class or stronger foe with ease. When in use, it is signified by a spark of white reiryoku discharge that snaps between the teammates, following by their eyes all glowing with a faint hue of white spiritual energy for a brief moment.

However, despite its vast benefits, the multi-soul resonance technique requires immense practice to perfect. Those involved in the team must be able to communicate souls together and not attempt to impose their will over another. At all times, they must maintain a balanced level between their souls, or else the multi-soul resonance shatters and falls apart. In addition, the concentration and practice required to preform the technique is proportional to the amount of spiritual power that each user possesses individually, the differences between their personality and mannerisms, and also in regards to the number of teammates preforming the multi-soul resonance; with greater numbers requiring an even greater level of concentration. Despite this, it has been often stated that the multi-soul resonance has no limit in potential, meaning that it is theoretically possible for an almost indefinite number of fighters to use it at a single time, reaching infinite heights of power. It seems that another factor involves whether or not the Shinigami are utilizing their Zanpakutō's release states, which actually equates to an additional user entering the formation and thus a greater effort is needed to execute the multi-soul resonance properly.

Behind the Scenes
This article and the theories behind it were inspired based on conversations with Void, Njalm, and Chrono. Certain elements were also derived from a concept with a similar name from the manga and anime, Soul Eater.