When played backwards, the star of the ad is actually Tilda Swinton’s Ancient One, who tells Stephen Strange to “open your eye” and that “you can become so much more.”
We also get the on-screen text in reverse, which ultimately says the same thing, just in a different way.
Leave it to Reddit to figure this one out, specifically user broganisms, who did the reverse work for the rest of us.
Both versions of the trailer end with Benedict Cumberbatch’s voice telling us that “this is just the beginning.”