An arrow is shot from heaven and hits right between your mouth and your mustache to correct your crooked smile. It also straightens your teeth and the crooked cut of your mustache too. Who told you that looked good? Well, it doesn't matter. The heaven arrow corrected it for you.
These heavenly correcting arrows actually come from きょうと. They hate seeing people with crooked mustaches, so they shoot them out on the regular. It just looks like they're coming from heaven because they fall out of the sky.