In this trailer we clearly see Sylveon standing there. Notice how the leaves are falling down, or not getting blown to a side? So if there's no wind, then how does it's ribbons stay floating? The only time they change is when it attacks, and then it reverts back to its normal position.
Basically the ribbons are an extension of its body, and Sylveon can keep them floating because it's a flying type.
Just a thought.
http://i.imgur.com/xWWX5xm.jpg http://i.imgur.com/tMhFpCD.jpg Thanks Fellwolf and L4DHunter! <3