Relevance theory

Relevance theory explains why a word or a phrase can convey much more information than its literal version. For example, imagine you own a really bad car - it breaks down often, looks ugly and it can barely hold itself together. You swear you'd demolish it yourself if only you had money to buy a new one. Your neighbor asks what you think of your car and you say "Well, it's no Ferrari". What you communicate is very little, but they understand the message.

The reason we can convey more than we say is self-assembly. When something is being communicated to you, you add relevant information from the context to that message (who said it to you, how they said it, who else is there, what has been said before, previous experiences...) until you arrive at the final message. So only a small part of the intended message needs to actually be said explicitly, everything else can happen due to self-assembly. Just think of an inside joke - one word can be enough to convey an entire story.

Once you understand this effect, you have a framework to understand different types of communication.

For example, a good joke gives away a strong enough connection that you understand what is meant, but the connection must be weak enough as not to ruin the punchline. A joke where the connection is too strong isn't fun because you can guess the punchline; there is very little self-assembly required by the receiver of the joke.

A good aphorism (or good advice) gives away a practical enough idea that you can apply it to your own experiences, but it must be abstract enough that you come up with the application yourself. Consider very wise advice: it's never straightforward "do-exactly-like-this" communication, you usually need to decipher the message, to construct your own meaning of it. It's more impactful to make you realize what to do yourself than to tell you exactly what to do.

In advertising, there is an incentive to communicate the most with the least amount of information or time. The bigger portion of the message can be outsourced to the context, the better for the advertiser, since now the receiver of the information does more self-assembly in their heads. This leads to a sort of mental IKEA effect where they'll enjoy your message more, now that it isn't pushed into their heads, but when they themselves are a co-creator of that message.

Generally, the smaller the ratio of what is communicated to what is understood, the better the aphorism, advice, joke, tagline or ad.

• Wikipedia
• Inception (Planting "seeds" into someone's head. Plus, it's a good movie)