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.