Ansatsuken is a fictional martial art used in Street Fighter. By tapping into one's "ki," an Ansatsuken user can channel the energy known as "Hadou." Special moves associated with Ansatsuken include the Hadoken (fireball), Shoryuken (Dragon Punch) and Tatsumaki Senpuu Kyaku (Hurricane Kick).
Over time multiple versions of Ansatsuken have emerged with the original purpose of all being assassination. The first division began with the creation of Ansatsuken in China, the original version of which is still practised by Gen, which was then brought to Japan and changed over the years to become the more commonly used version in Street Fighter games by Ryu and Ken.
The second division began after many years of Japanese Ansatsuken tutelage. The excepted practice taught by the last traditional Anasatsuken master Goutetsu was that in order to master the art, users must rigorously train their body and mind their entire lives to tap into the Satsui no Hadou technique without losing themselves to its power.
Goutetsu's students, the brothers Gouken and Gouki (Akuma) both disagreed on the traditional teachings of Goutetsu, but for widely differing reasons. Gouken argued that such murderous intents associated with the use of Satsui no Hadou shouldn't be relied upon and that a more peaceful variation could and should be created for the betterment of the art and its users. Gouki on the other hand looked down upon the previous generations' fear of Satsui no Hadou and planned on not only mastering Satsui no Hadou but becoming one with it. He argued that If those who attempted it died or lost their minds in the process then they were simply weak.
Both succeeded in their efforts. Gouki became one with the Satsui no Hadou, unleashing a new terrible power, the Dark Hadou. This Hadou transformed Gouki's body, giving him a demon-like appearance, earning the name Akuma. With his new-found powers Akuma mastered the ultimate technique of the Satsui no Hadou, the Shun Goku Satsu AKA the Raging Demon. An instant death technique which Akuma demonstrated during his battle with his former master, Goutetsu, killing him.
Gouken on the other hand created Mu no Ken Hadou, the Power of Nothingness. Through spiritual and mental refinement, Gouken learnt to channel Hadou through the purity of mind, the complete opposite of Satsui no Hadou.
Gouken taught Mu no Ken Hadou to his students, his adoptive son Ryu and his friend's son, Ken Masters. Dan Hibiki was taught by Gouken for a time but was banished, it was discovered that Dan's intentions for learning Ansatsuken was in order to avenge his father by killing Sagat. Dan attempted to create his own fighting style based on his uncompleted Ansatsuken training.
Characters who use Ansatsuken are commonly referred to by fans as "shoto" characters, as they were once thought to be users of Shotokan Karate. The term often extends to any character who uses uppercuts, fireballs and forward jump kicks (e.g. Sagat) whether or not they are Ansatsuken users in the story.
Ryu, Ken and Gen appear in the 1st Street Fighter game, though Ken is merely a player 2 palette swap for Ryu, they play the exact same way. Special moves such as the Hadoken and Shoryuken were notoriously hard to execute and the commands for them were not provided by the game. But successful attacks with either resulted in victory, such were the power of the moves in the 1st game, made before Street Fighter II would start competitive fighting game play and combo system.
In other media
Various Street Fighter content in other media has been created since Street Fighter II. Many of them merely borrow aspects from the games and thus Ryu and Ken's Ansatsuken special moves are rarely explained, if used at all.
In the critically panned 1994 Street Fighter movie, Ken is played by Damian Chapa and Ryu is played by Byron Mann. Ryu and Ken are depicted as con-artists and do standing hand and kick motions resembling the Hadoken, Shoryuken and Tatsumaki with no explanation.
2014's Street Fighter: Assassin's Fist, created by Joey Ansah and Christian Howard depicts the back-story of Ansatsuken, focusing on the time periods of Goutetsu (Togo Igawa) teaching traditional Ansatsuken to a young Gouken (Shogen) and Gouki (Gaku Space) and an older Gouken (Akira Koieyama) teaching the Mu no Ken Hadou variation to Ryu (Mike Moh) and Ken (Christian Howard) while Akuma (Joey Ansah) emerges.