Power of Nothingness

The Power of Nothingness is a power used by Master Gouken to stop Akuma from taking Ryu during the sealing of Ryu's Satsui no Hadou.

Description
The Power of Nothingness is a state of consciousness transcending ordinary perspective, born through spiritual and mental refinement. Bearing common traits with states of being within Buddhism and martial arts, it is the power to act without being attached to emotions and thought, free of fear, anger, and ego, to draw upon the contents of one's heart intuitively without obstacle and hesitation, and to be aware of the world and to know one's place and meaning within its vastness, to flow with all of creation without the need of worry, desire, and doubt.

It is the complete opposite of the Satsui no Hadou, and can only be used by a warrior who has a pure heart of goodness and has replaced their hate and rage with compassion, mercy and justice, as well as embracing the peaceful and spiritual aspect of the art to their fullest extent. Just like the Satsui no Hadou, it makes a person unbelievably powerful, granting invincibility (spiritually and physically) to many attacks, as well as sealing off evil power and making the user a stronger fighter.

Gouken
Gouken has practiced with this power throughout his years of training with Goutetsu, and after he had completed his training and left his master to begin his own dojo. He is the only one who has used this power in battle in the games, as shown by his 'Shin' form.

Ryu and Ken
Ryu and Ken also possess this power, as shown below, but have not mastered it to the same extent.

Go Hibiki
Go seems to be able to use the power of nothingness even better than Gouken. In Pocket Fighter, he can be summoned by Dan to do a Jun Koku Satsu/ the only good intent/ non Satsui No Hadoi requiring version of the Shun Goku Satsu. Also, Jun Koku Satsu translates as Pure Heaven Murder.

Street Fighter IV: The Ties That Bind
Near the beginning of the animated movie Street Fighter IV: The Ties That Bind, Ryu said to himself that he sought a power better than the Satsui no Hadou, but has no clue what it is, all the while arguing with Akuma's voice in his mind.

Later, when C. Viper refused to tell Ken where his wife was and attacked him with an electric fist - that had sent Ryu reeling even while blocking it - Ken awakened the Power of Nothingness, repulsing C. Viper with a single block while keeping a resoloute and justice toned face. C. Viper retreated and reported back to the CIA; her undercover identity as S.I.N. agent 'Maya' was useless now that Ken could defeat her easily. C. Viper noted to Ken before she left that while Ryu using the Satsui no Hadou was impressive, Ken was "no slouch" with the new power he showed, relatively. Ken realizes after reuniting with his wife that he has a power that comes from protecting the ones he loves, and that he would never need the Satsui no Hadou, even if he had the talent for using it.

Near the movie's climax, Seth was trying to force Ryu to use the Satsui no Hadou. Seth questions why Ryu, a man who seeks power, keeps the Satsui no Hadou's power locked inside - as he described it, power was the natural human inclination to trample on others and the natural destruction of human life. When almost beaten, Ryu finally realizes what the power he seeks really is, which is the natural 'destination' of human life, and the courage to stand up against evil and win. When Seth laughs and argues that power is power and that there is no difference, Ryu shows him the difference and awakens the Power of Nothingness within his art, stopping Seth's near invincible punch with one grab. Exceeding Gouken's trained power and Ken's instinctive power of this ability, not only does he gain incredibly enhanced strength and near invincibility, his eyes suddenly go bright blue, and his body gives off an abnormal energy.

At first, his ki goes off the charts when anaylized just like the Satsui no Hadou gave him, but then its power shows an infininte amount of zeroes, symbolizing invincible nothingness. The skin on Seth's arm where Ryu grabbed it starts burning away, as if the power itself with Ryu's talent not just seals evil, but deteoriates and destroys it. As Seth charges at Ryu in fury and shock, Ryu summons all his power and concentration at once, and fires a purified Metsu Hadoken at Seth with more power than any Hadoken he had ever fired before. The beam blasts Seth 50 feet away, not only leaving his body completely beaten from the impact, but also causes his entire body to go into muscle spasms, which leave him nearly completely helpless after they subside.

Ultimate Marvel vs. Capcom 3
In Ultimate Marvel vs. Capcom 3, Ryu has access to a powered up mode called Hadou Kakusei (Surge Awakening) which is reminescent of the Power of Nothingness; it envelops him in a blue aura and turns his eyes a pure white. While his voice becomes rougher, similar to Evil Ryu, he is in complete control and all of his Hypers are powered up exponentially while his speed and attack power increase. His Shinkuu Hadouken gains the ability to reflect off of surfaces and because of its sheer size, nigh-impossible to evade. The Shin Tatsumaki Senpukyaku becomes a near screen-covering hurricane that seems to generate a storm and the Shin Shoryuken covers more range.

Trivia

 * While not named or stated, the first instance of the Power of Nothingness being shown was actually displayed by Ken in the US cartoon when he taps into it to defeat Akuma.