I've been playing Smash 4 quite a bit lately, and have seen online that some players are able to use Marth/Lucina's Shieldbreaker to propel them a substantial distance horizontally while in the air. Checking smashboards and the wiki confirms that Shieldbreaker was buffed to have a small amount of horizontal propulsion. My own experimentation that simply using shieldbreaker in the air produces a small amount of horizontal propulsion, however nowhere near the levels I've seen online. Does anyone know how to do this? I tried inputting a direction after starting shieldbreaker, much like Dedede's gordos require, but it didn't seem to do anything.
Marth/Lucina's shieldbreaker can be held down for longer to create a stronger hit (as demonstrated here). When the shieldbreaker is tapped in midair, you'll receive a small amount of horizontal propulsion. However, if you hold down neutral B in midair and charge the shieldbreaker fully, Marth will gain a large amount of horizontal propulsion. You don't need any other button inputs to get the boost.
Note that once you reach a full shieldbreaker charge, Marth/Lucina will automatically complete the attack (which is where you get the large boost). If you try to let go too soon in midair, you'll only get the small amount of propulsion. Also note that Marth/Lucina cannot jump high enough to perform this boost by the time they reach the ground on a level stage like Final Destination (You'll hit the ground before the charging is complete, so you'll throw a ground shieldbreaker instead).
Short and simple, hold down B and don't let go.
It's very useful for recovery if you get launched off to the upper corners. You might want to practice at a huge stage before trying it out in battle, because you can't just double jump and then do the shieldbreaker. You have to be really far up in the air to carry it to its full extent.