Persuasive techniques refer to the various techniques which are utilised in order to persuade the viewer to support the writer's opinion or to influence then to feel a certain way.
A metaphor is a figure of speech which is applied to an object or action to indicate a sense of similarity or likeness between them, although it isn't literally applicable.
It can be used as a persuasive technique as it can influence the viewer's emotions. As well as this, it can produce a powerful and memorable idea or image for the viewer, thus enabling it to be an effect communicator of both tangible and conceptual information.