Symbolism, Imagery, Allegory
Without Toby's magic words, the people would never remember how to fly. Abracadabra just wouldn't have worked.
In a sense, these magic words represent the cultures erased by slavery. After their enslavement, the African people weren't allowed to practice their own cultures or even speak their native languages. The story connects this idea to their power of flight, which evaporates when the now-enslaved people "could no longer breathe the sweet scent of Africa" (3). Later, their powers only return when "the words of ancient Africa" (21) are spoken to them. In both cases, we can see access to culture as essential to thriving.
Essentially, then, Toby is reconnecting the people to their stolen past. His magic words reawaken the people to the power that resides inside them. But some changes can't be undone, and "the words [...] once heard are never remembered completely" (21). These now-free people will never be able to go back in time and prevent their culture's destruction, but they will get a chance at a fresh start.