Hm. Given the shapes' likenesses to the Sun and the Moon, it's quite possible that at the point where they appear, the Pokémon are drawing power from their respective celestial bodies by whatever means. I couldn't really tell from the trailer whether the symbols were appearing in a cutscene or during battle. If the latter is the case, this close-up shot may be the initiation of each one's signature move.
Alternatively, it could simply be a visual effect added for no particular reason other than to look cool.