An example of how one might learn a dead metaphor.
Suppose someone hears the phrase Balls to the Wall for the very first time ever in this sentence:
> The deadline was approaching, so the team went balls to the wall.
The individual words – “balls”, “to,” “the,” and “wall” – don’t suggest much of a meaning for the phrase.
That’s not a problem because of context. The first part of the sentence is about an approaching deadline. An approaching deadline will likely activate brain Networks associated with risk or danger, both because of experience with physical objects coming at you fast and because the concept “deadline” is associated with the concept “missed deadline,” which has what scholars call a “negative valence 
” – that is, it’s associated with feeling bad. So networks associated with “bad situation” will be active as the second half of the sentence arrives.
In the sentence, the team did something in response, something associated with metaphorical movement. It doesn’t take much experience with deadlines to suppose that the response is to move faster and work harder. So networks associated with those concepts will activate. “Balls to the wall,” then, can be treated as just a token for those activations, requiring no interpretation like that of Propositional Metaphor.
For some people, that might be the end of it. The phrase “balls to the wall” temporarily activated some phrase-specific neurons, but they don’t undergo any physical changes that would make the phrase “stick.” There is not even a fledgling “balls to the wall” Network. In other people, the situation or sentence might be important enough that the phrase gets a little network that, when activated, tends to recursively activate (or help activate) the “move faster” and “work harder” networks. Because of context, there’s no pressing need for our hero to ask, “What does ‘balls to the wall’ mean?” And, over time, the “balls to the wall” network will get stronger as the phrase is used in more contexts, and it will more readily activate a network about a certain kind of all-out effort.
It has become a cache lookup. Cache Lookup vs. Calculation.