Soul Resonance (魂の共鳴, Tamashii no Kyōmei; Literally meaning "Resonance of the Soul") is a special technique used by Meisters and Weapons, in which, they synchronize their Souls wavelength. By doing so, they can release highly powerful techniques and, more often than not, change the tide of a battle.
Resonating Souls is the foundation of Meister and Weapon relationships. Resonation is the act of linking Soul Wavelengths. A meister matches his/her soul wavelength with their Weapon's wavelength. When the wavelength's are synched, this results in a Super-Technique unique to each weapon and meister pair. Maka naturally has the Witch Hunter technique from her mother, so when she resonates with Soul, her mother's ability awakens.
Resonating requires both Meister and Weapon to be calm and of one mind. Should one of the souls not be in sync, techniques can not be performed. If a technique is being used and one soul loses focus, the technique will lose power or direction, as seen with Maka losing her footing when she lost focus while using Witch Hunter against Sid. Common reasons for souls not to sync are fear, arguing, stress, etc. In addition to those factors, Maka couldn't match wavelengths with Soul, much less hold him, due to the "Special" training Maka asked Stein to put them through, the arguments they went through caused their wavelengths to become out-of-sync.
It is possible to resonate with more than one soul at a time, but it is noted to be a hard technique to control and use effectively. Kid and Kilik are the only two Meisters that have been noted to wield and resonate with more than one soul at once. When they are using soul resonance , their soul connects with each other to make a "Soul Resonance".
Soul Wavelength Synchronization
Soul Wavelength Synchronization, also known as Chain Resonance, is a kind of resonance that involves a team of Meisters matching their wavelengths with one another in order to empower each other. It is extremely difficult to maintain, unless all the team members can easily keep their souls on a balanced level. There is no set number to the amount of meisters required to perform the Chain Resonance, but it is more powerful yet more difficult in a larger team.
Bonds are a method of resonating exclusive to that of family members and any immediate/direct relative such as that of a parent or sibling. No matter the circumstances, it seems that a family member can always wield another should one be a meister and another a weapon. Even Soul Resonating attacks are possible. However, despite that, resonating between family members isn't as strong due to a similarity in wavelength.
Other forms of Resonance
Screech resonance is the form of resonance that Crona and his/her Demon Sword Ragnarok use. When initiating the resonance, the mouth that appears on Ragnarok (as a weapon) opens wide screaming out a loud sound wave. This sound wave makes Ragnarok vibrate like an electric saw, increasing his cutting power, and can also radiate through the air causing internal damage to an opponent and even other weapons. This is shown when Soul felt pain and was injured even in weapon form just from being in contact with Ragnarok. Beside the piercing sound wave, this resonance is just the same as a normal Soul Resonance.
Main article: Soul Menace
Soul Force is a form of "self resonance" with one's soul, meaning that the user's own soul becomes powerful enough that it is not restricted to using the amplification of a weapon in order to attack. This allows the user to use attacks that cause damage to a target internally.