From SmashWiki, the Super Smash Bros. wiki
Wing dashing is an advanced technique that can only be used by Pit. It is the input of four moves in rapid succession to produce a motion similar to that of wavedashing, although it is more difficult to make a wingdash go a similar distance. The summary and relevant SmashBoards thread can be found here.
To wingdash, the player must input a short hop or a jump, then activate Wings of Icarus. While using Wings of Icarus, he/she must press left or right based on the desired direction, then perform a down aerial.