Tenma Kuujin Kyaku (天魔空刃脚, lit. Sky Demon Air Blade Leg) is one of Akuma's special moves/command normals. Akuma performs a dive kick at the apex of his jump with his leading foot's outer metatarsal portion like a blade. This attack is a well-known command normal that is a staple diving kick in fighting games to come.
In previous MVC games, Akuma has it as a special move that can be done any time in midair and does multiple hits with a air combo/aerial rave finish effect on midair foes, as well as being able to knock down grounded foes. At a cost however, its surprise element is hindered by having a bit more startup time. In MVC3 it was changed back into a command normal like in past Street Fighter games with no special hitting effects, and its startup speed was maintained making it easier to use for intercepting anti-airs.
It's main usage is to beat most anti-air tactics (especially with most normal attacks angled well upward) and to cause a safe jump-in leading into a combo. Usually the hitbox of the attack if positioned well enough can enable it to crossup (especially since Akuma/Gouki can reach the ground very quicky while diving, being able to quickly act afterwards), though some attacks with a better steep angle control can do this task better.
Diving Rocket KujinkyakuEdit
Diving Rocket Kujinkyaku is Cyber-Akuma's version of the Tenma Kujin Kyaku. Just like Akuma's Tenma Kuujin Kyaku, it doesn't do more damage than Akuma's but its faster, has more priority, and Cyber-Akuma can recover quickly from it and continue his combo.