Definition of flattens:
(v) :
(transitive) To make something flat or flatter.
(v) :
(transitive) To knock down or lay low.
(v) :
(reflexive) To press one's body tightly against a surface, such as a wall or floor, especially in order to avoid being seen or harmed.
(v) :
(intransitive) To become flat or flatter; to plateau.
(v) :
(music) To lower by a semitone.
(v) :
To make vapid or insipid; to render stale.