Move description courtesy of Suka PassThis kick begins as a high outside spin kick, but ends as a heel drop.
For an outside spin kick, the path of the kick is compact, and rather than obtaining power from the spin kick itself, the importance of this move lies in sweeping away the opponent's guard against hte main attack from the heel.
The name Thunder Kick is said to come from the High Thunder Sword stance of the Yagyu Shinkage Style of Kendo Swordsmanship. It is known from written records passed through time that at the end of the Warring States Era, the Hazuki Style and Yagyu Shinkage Style enjoyed close relations.
The Yagyu Hyogonosuke of the Yagyu Shinkage Style is known for devising the High Thunder Sword (also called Upright Body), which involves raising the sword high above the head and maintaining a straight posture, impossible when wearing armor. It was developed form the so-called Kaija Art of Swordsmanship whose practitioners wore the armor of the Warring States Era and maintained a low waist position. As a result of the change, the length of the stroke and its power increased dramatically.
It is probably a fact that, despite the differences between a sword stroke and a kick, because the concepts were similar, the Hazuki Style devised the Thunder Kick by making reference to the High Thunder Sword of the Yagyu Style.
When mastery is attained, it it possible to follow other kicking moves with the Thunder Kick.
The Thunder Kick is more commonly known as an axe kick, or Kakato Otoshi Geri. This technique is common in martial arts such as Karate (particularly Kyokushin Karate) and Tae Kwon Do. Kyokushin karateka Francisco Filho knocking down his opponent with an axe kick in competition.
This technique, though relatively common in Kyokushin and TKD competitions, is rarely seen in kickboxing and MMA. Axe kicks require an extraordinary level of flexibility and strength to land with any meaningful power and have a relatively short effective distance. If the axe kick falls short or is interrupted in its path, the user is open to a counter or takedown.
In spite of this, there are fighters who have been able to use axe kicks successfully at the highest levels of kickboxing and MMA. The most famous fighter to utilize the axe kick is the late Andy Hug. Coming from a Kyokushin background, Hug's unorthodox kicking style saw him winning a K1 World Grand Prix and 2 runner-up finishes. Though he never KO'd any of his opponents with the axe kick, the mere threat of his axe kick was enough to make his opponents respect it. To avoid Hug's axe kick, the opponent had to either create distance or move their hands out of their guard to defend against the kick. Hug capitalized on this reaction by feinting his axe kick setup. As his opponents raised their hands to block the axe kick, Hug flurried with punches at their now-unprotected face. Andy Hug landing axe kicks on kickboxing greats Ernesto Hoost and Mirko Cro Cop.
The description for the move mentions that a master of the thunder kick can use it as a follow-up after other kicking techniques. In-game this is represented by Ryo throwing a roundhouse kick to the midsection, followed by the thunder kick. Strikeforce MW champion and Sanda World Champion Cung Le shows this exact setup in his book. Le throws a roundhouse kick to draw a leg check from his opponent. Le then throws the axe kick. As the axe kick's setup looks similar to the roundhouse kick, the opponent raises their leg to check, only for the axe kick to go around and crash down on their head.
A real-life example of this setup occurred in the rematch between Mark Hunt and Mirko Cro Cop. Cro Cop, infamous for his lethal roundhouse kicks to the head and midsection, had landed several hard body kicks against Hunt. Cro Cop then throws an axe kick instead of his trademark roundhouse. Hunt raises his leg to check the expected body kick, only to eat an axe kick instead.
While no fighter in MMA has used axe kicks to any great effect like Hug has in kickboxing, there have been some interestin g applications of the move. Kicks to a downed opponent are illegal in most MMA rulesets, but kicks to the body are still legal. MMA GOAT Georges St. Pierre utilized an axe kick to hurt a grounded Jason Miller. The idea clearly resonated with GSP as you can see him teaching Raymond Daniels the same move in order to exploit the "pit" used in the Karate Combat promotion. GSP axe kicking Jason Miller, who was attempting to lure GSP to the mat so Miller could use his jiujitsu skills.
Yagyu Shinkage Style
The Yagyu Shinkage Style mentioned in the description is a real style of Japanese swordsmanship. I can't comment on how accurate the description of the tactics used in the Yagyu Shinkage-ryu are having never trained in it, but at a layman's glance, there seems to be some truth in it. Videos of the Yagyu Shinkage style seem to show its practitioners using a jodan (sword held upwards and used to strike high targets) stance. One of the reasons why axe kicks are effective and difficult to block is that they exploit the vertical ceiling-to-floor line. While strikes in the vertical axis are quite common in swordsmanship, they are rare in unarmed fighting as few attacks use this line. A Yagyu Shinkage-ryu demonstration
