I swear, the attachment to Sheik is overwhelming here. Anyway, I'm gonna tackle this under the assumption that the Phantom Knight is her new Down B and how it might work.
Using Down B, Zelda summons a Phantom Knight to her side. Doing Down B again and holding B will allow the Phantom Knight to move. Like possibly Luma with Rosalina, you may be able to keep Zelda stationary by using the down-forward and down-back inputs so the Phantom Knight can move on its own. By releasing B, we have Smash's first ever instance of Negative Edge. Zelda will order the Phantom Knight to strike.
The above theory would explain why Zelda does that same pose in one screenshot with it and without it in another - the Phantom Knight is simply off screen. Meanwhile, the AT summoning pose and this one are very different in body position. While it is possible that the pose may have been redone. Nothing necessarily suggests one way or the other.
As for an additional theory, Din's Fire has been revamped to where the fireball (or possibly more) do not explode automatically. Rather, you set it/them and trigger their explosion using her side taunt which just so happens to feature a fireball in her hands.
These two theories would allow Zelda to have that much needed zoning capability she has been so desperately needs with some influences from Carl Clover and Mu-12 of BlazBlue fame - because Rosalina is totally a Smash version of Guilty Gear's Eddie.
EDIT: Holy Shinto! What if Zelda's Jojo pose is her controlling the Phantom Knight?! Look at the pose and then then the pose with the PN striking:
Judging by the pictures, it would be easy for Zelda to transition from those two poses seamlessly.