Each spell is graded on a scale from 1 to 99, the latter being the most powerful and the most difficult to perform. The scale is a measure of difficulty, as there can be multiple spells which share the same level. To use Kidō, a Shinigami must recite the specific incantation for the spell, which is often long and requires a few seconds to speak.
This weakness can be negated by Soul Reapers capable of using Eishōhaki, a technique that releases Kidō without a spirit chant. While it decreases the time needed to release the Kidō, it drastically weakens the spell.Bleach Official Character Book Souls, While anyone well versed in Kidō can use this technique, it is most effective when done by a skilled practitioner, otherwise the spell could utterly fail (ie: the spell explodes in the user's face). The most skilled experts are capable of using this technique to amazing effect.
Another method of circumventing the necessity of uttering an incantation for a Kidō is something called Kōjutsu Eishō, where the incantation is recited after initiating the spell to power it up. So far, only Hachigen Ushōda has been seen using this technique, in order to reinforce the nature of his Ryūbi no Jōmon during his fight against Baraggan Luisenbarn.
The power of a spell relies on the power of the user, as even a low-level spell can be utterly devastating when utilized by a high-class Soul Reaper.
Kidō can also be used for practical purposes as well. Renji Abarai has demonstrated that a kidō spell can be used to light a dark hallway if the spell is tweaked in the right way.
There are three main types of Kidō: binding spells, destruction spells, and healing spells.
Bakudō: These are a broad category of defensive spells which block/repel attacks or freezes enemies in place. This class of spells include Kidō Barriers & Seals.
Hadō: These are offensive spells which harm an enemy or cause damage to objects.
Healing spells: These do not have known names, numbers, or incantations to cast, and simply heal the target. As the user holds his/her hands above the patient's wounds, his/her palms glow with green spiritual energy to facilitate healing. The mechanics of this healing technique involves the replenishment of the patient's Reiatsu first, then using the body's Reiatsu with the healer's Reiatsu in order to heal the patient's body.
Though Kidō is commonly used by Shinigami in combat, it has several weaknesses that make it difficult to use for that purpose. All of the spells have fairly long incantations and require concentration; because of this, the person using the Kidō leaves themselves open to attack during the duration of the incantation.
In addition, if an opponent is familiar with Kidō, there is the possibility of them identifying the spell by its incantation before it can even be used. However, highly skilled Kidō users can use various techniques to counter these weaknesses, such as Eishōhaki, Kōjutsu Eishō and Nijū Eishō, which involves mixing the incantations of two or more Kidō spells together to save time and confuse ones' opponent.
A rarer use of Kidō is mixing various spells (such as Bakudō and Hadō together) for unique effects, which is the mark of a skilled user.
Kidō is commonly seen used in instances of surprise attacks, rather then all-out assaults, being most effective when one's opponent doesn't see it coming. Kidō is also usable with just the spell name and no incantation or spell number.