@The Double U
Yeah. Derpy was behind Twilight. But you can see Rainbow Dash’s blurry rainbow right in front of Twilight because Rainbow Dash saved her from being petrified by Tempest. And Rainbow Dash would have been distracted or caught by the Storm King’s minions along with the rest of the Mane Five and Spike or be impaled in the heart or chest by one of the Storm Creatures’ spears or get petrified by Tempest if Rainbow Dash flies straight towards her for doing this but gets caught by Applejack’s mouth and breaks free and charges straight towards and said “What did you do our princesses?!” Just like what she said in the pilot of the show and the four other ponies and Spike would attack Tempest and get petrified by her if Tempest succeeds. And the movie would have featured the Storm King’s troops chase the Mane Six with not just spears and shields but also obsidian orbs too while they’re chasing them in Canterlot or in Klugetown. And maybe Tempest would have successfully turn Twilight into a statue instead of caging her if she finds in either Klugetown, the Sea of Clouds, or Basalt Beach, or on the ship, or back in Canterlot in Equestria. And this would ultimately let to the Storm King said to Tempest, “Why is this one still moving?” And Tempest tells her that Twilight have failed friendship and let her friendship failed her by making the Magic of Friendship broke apart.

@The Double U
It wasn’t Derpy who saved Twilight for being petrified by Tempest. It was truly Rainbow Dash who saved her. And maybe what if the rest of the Mane Six could be together as Twilight is about to be hit just like in Equestria Girls, then maybe all six ponies plus Spike would be petrified by Tempest or maybe someone takes the bullet to save Twilight.

@northern haste
Ohhhhh, okay.

I was also paying attention to Derpy running towards Twilight.

@The Double U
You can clearly see rainbow dash’s rainbow in the scene

Everytime I watch The Scene where Derpy saves Twilight, I keep hearing Rainbow Dash’s Voice saying TWILIGHT! Making it sound like Rainbow saved Twilight, but Derpy actually saved Twilight. It should sound More like Derpy, not Rainbow Dash.

@Background Pony #944D
Thus buying Twilight Sparkle just enough time to escape, thus setting the rest of the movie plot in motion.

Tempest: Grubber, get her now!
Grubber: GUYS, WE GOTTA GET THE LAST PRINCESS!!!

@Background Pony Number 17
No she didn’t. She was in front of Twilight, but gets caught in the crossfires by that obsidian orb.

And that is how Derpy saved Equestria.