Kuuenbu

"With the correct timing, higher and further jumps are possible while airborne. Kuuenzan may be activated while attacking in the air."

- In-game text

Kuuenbu or Kūenbu (空円舞), is one of the two special techniques that Zero learns after defeating Split Mushroom in Mega Man X4. This technique allows Zero to jump a second time in midair -- by pressing the Jump button a second time -- allowing him to jump even higher to gain access to otherwise out-of-reach areas (provided that the player does not dash-jump beforehand).

Similar techniques
In Megaman&Bass Bass can perform a second jump, even after performing dash jump. However the speed is always reset to walking speed, if doing so.

In the 2nd half of the Mega Man Zero Series ( namely Mega Man Zero 3 and Mega Man Zero 4), Zero can equip chips to allow him to perform a second jump in mid-air. Unlike in the X Series, Zero can dash jump and even keep the speed from the dash while doing so. It effectively makes the need of a horizontal air dash redundant.

To Note: Megaman Zero 3 and 4 are the only games, which allows Zero with the usesage of chips and cyber elves to completely replicate both Kuuenbu and Kuuenzan at the same time.

Strangely enough in Mega Man ZX and Mega Man ZX Advent Megaman Model ZX and Model OX cannot perform a second jump in mid air.

Trivia

 * It is possible that this "move" indirectly appears in Tatsunoko vs. Capcom, with the obvious reason of why Zero can double jump in the game.
 * However, in Marvel vs. Capcom 3, he oddly loses his ability to double jump as a balance measure from TvC.
 * From X4 onwards, Zero can use this attack and the Hienkyaku by default.