Kuenzan/Kuuenzan (空円斬, lit. Air Circling Slash) is one of Zero's command normals in Marvel vs. Capcom 3. Zero performs a forward somersault slash in a tucked-aerial roll which acts as his j.H, and its command normal variation does it the opposite direction. There is no distinct difference between them other than the speed, where the backwards Kuuenzan is actually faster than j.H.
This move originates as a special extra from his Kuuenbu (Air Waltz) Command Art from Mega Man X4, and since that game, the double jump ability along with a somersault slash variation since the Kuuenzan has been available for Zero in many of his playable appearances, making the Kuuenzan's motion one of his signature techniques. The forward somersault slash has gained many variations such as the C-Sword/Mikazukizan (Crescent/Three Day Moon Slash) in X5, Ensuizan (Circling Water Slash) in X6, Rasetsusen (Swirling Temple Spin) in X8, Zero Skull Crush/Rei Shiki Kabutowari (Zero Style Helmet Splitter) in Command Mission, and some of its basic variations in the Mega Man Zero games.
For Zero's playable vs. Capcom appearances in SvC Chaos, Tatsunoko vs. Capcom and Marvel vs. Capcom, he has had a variaton of the Kuuenzan as a jumping hard attack ever since (SvC Chaos has it as j.C/Hard Punch, and both TvC and MVC3 have it as j.H), and all versions do multiple hits of three like the original Kuuenzan along with being great for crossups due to their circular hitboxes and good hitstun, making them strong for attack approach via the frame advantage on hit or block when timed right. Tatsunoko vs. Capcom's version acts as Zero's main air combo finisher, till he gains a new one in MVC3.
In both TvC and MVC3, Zero has an alternate variation of the Kuuenzan done backwards in midair as a command normal; this is a direct call back to how Zero can change directions during many of his somersault slash variations quite freely in much of the Mega Man X games, especially due an attack such as a rolling somersault slash via its moderately-long duration (despite only somersaulting once in X4 and X5). In TvC, Zero would have a startup animation of turning around before pulling off the backwards Kuuenzan, while in MVC3 the startup is no longer there as it seems almost faster than his j.H.
Regardless, either version is amazing for crossup potential, but it should be noted that as a major nerf from its TvC version, it can no longer be cancelled into anything but his j.S in air combos, so Zero players mainly have to try hard to link from the ending of their j.H chains. This alone tends to make Zero's more creative aerial combos rather strict in timing, as well as the blockstring pressure from it since it can no longer cancel into special moves like it did in TvC. The hitbox if timed wrong, can also have Zero's hurtbox exposed during certain intervals of the rolling slash, which also allow quick attacks to intercept and counter-hit him out of it if either version of Kuuenzan is spammed recklessly for approach.