For that particular shot, I would guess either an assistant, or maybe the model flipped her head and upper body to give that effect.
With that amount of hair, it would take a pretty strong fan to make it move that much - and the skirt doesn't show any sign of blowing/flowing, so I would guess that it's not a big fan. Similarly, since the skirt isn't flared out, it's not a full-body twirl.
Anyway, there are a variety of ways to do it. Powerful fans are a lot of fun but a little unpredictable, and can wreak havoc on delicate costumes.