This isn't an advanced technique, its standard input buffering and is done in many, many games. I add it to every game I've programmed. Its also not a new discovery - my group noticed this weeks ago, but found it to be a negative more than anything. It means, for example, if you double-tap a direction while blocking, you will roll twice. In Melee you would roll once, so tapping the direction multiple times was a habit we had formed to make sure the roll would go through. Same thing with many other moves. Now this habit works against us, causing us to perform moves we didn't mean to because the move has been queued up because we pressed the input more than once. One member of my group hates this feature so much he stopped playing Brawl altogether because his character kept doing moves that were queued up that he did not expect.
The worst one is turning around just after doing something when you didn't intend to turn around, but it just queued up a tap in the opposite direction from a previous move, and then you do a nuetral attack and you are facing the wrong way. I kept shooting Lucario's Nuetral-B the wrong way because I would dodge backwards then fire, and the dodge backwards I would tap twice out of habit and when the dodge ended, the second tap would cause him to turn around and then he'd shoot the wrong way.
Its nothing to get excited about and doesn't allow you to do anything you couldn't do before, its just a control convenience (once you adapt to it) that you don't have to wait for the precise moment to do another action, you can "queue" the action a bit early and it will go through when the opportunity comes up.