Hienkyaku (飛燕脚, lit. Flying Swallow Legs) is one of Zero's special moves in Marvel vs. Capcom 3. Zero dashes forward or upwards depending on the button pressed. In conjunction with the Hyper Zero Blaster, it can create some ambiguous crossups.
On Ground: Short distance forward
On Ground: Longer distance forward
On Ground: Upwards and then downwards while covering a bit of distance
In Air: Straight down
Originally hailing from Mega Man X4 (though Zero had this ability prior X4 in Xtreme 2 chronologically), this ability enables Zero to air dash. Like with his Kuuenbu passive ability, Zero's air dash has remained a recurring ability in most of the X games onward.
In Tatsunoko vs. Capcom, the visual effects of the ability were simply Zero dashing while leaving red silhouettes behind. MVC3 adds in the blue jets on Zero's feet and increases the speed of the move. Originally it was the only special move in TvC to not be powered up by Sougenmu as it had no offensive capabilities. The grounded H version of the move is based off of players in the Mega Man games being able to jump-cancel a dash, gaining extra height and horizontal air movement; thus, a ground H version of Hienkyaku is basically a jump-cancelled dash in that manner.
Usually, this move possess little-to-no invincibility, hindering its possible evasive usage and limiting it to primary use of mobility and crossups. The H version works as a minor overhead setup as attacks can be done at a certain point of the dash's end to cancel it into an attack of one's choice, making it useful for surprise attacks. However, its main touch since TvC (but not so much in MVC3) is making use of the H version in midair (originally L version in midair in TvC) to dash downward quickly to the ground during an air combo, especially to reset movement options and to quickly re-juggle the opponent for longer "pringles" combos, especially in the corner. This trait combined with a few others is what lead to Zero being rather overpowered in terms of both damage and meter building, due to strong combo potential.
In MVC3, Zero doesn't see much usage from this move anymore, mainly due to being eclipsed by the potential of his Raikousen and Sentsuizan. But it still has its usages during some mobility moments and if one needs to reach back to the ground quickly for a re-juggle during an air combo.