"The flux capacitor--it's what makes time travel possible." When to keep world-building explanations short

Gosh, I really think I'm quite clever sometimes. But what about those situations where the readers (audience) can be told, and they feel completely satiated and entertained by not going into the nuts and bolts. There are tons of examples: How about all the James Bond gadgets? Especially the fireball-shooting pen (Never Say Never Again, I think). And then there's the opposite, like the information dump from Morpheus to Neo in The Matrix. I ate them both up.

Why? What made me want to know everything in one situation but tune out the rational part of my brain in others? More importantly, how do I know when to keep world-building explanations short versus totally geeking-out?

I believe this is a writers question rather than a world builders since I'm not asking HOW to build a world.

Answer

I think there are two basic reasons for describing anything in fiction.

One is to give sensual pleasure in its own right. There are all sorts of sensual pleasures that prose might convey, from the erotic to the gastronomic to the social. Tom Clancy's loving descriptions of really big machines with really big guns are designed to titillate those of certain tastes. The depiction of suffering and injustice in the works of more liberal minded authors is designed to give a frisson of self-righteousness to the reader. Harry Potter depends heavily on the child's pleasure in the fantasy of being special and powerful, of being rewarded for extraordinary gifts where others would be scolded and sent to bed.

The sensual appeal can be very strong, but it is also very individual. What entrances one reader will repulse another.

The other reason for description in literature is to support story. This is about what we need to know to follow the story and to feel fully immersed in the story.

The reason backstory dumps are so often a problem for writers is that they often come before the reader cares about them. Unless they are well enough written to give sensual pleasure, and the reader is receptive to that particular brand of sensual pleasure, then the description is tedious if it is not relevant to the story as the reader is currently experiencing it.

Back to the Future is a fish out of water story. How the flux capacitor works is not story relevant, and would not give sensual pleasure to most of the intended audience, so it is not detailed in the movie.

The Matrix is a much more philosophical movie. It deals with the ancient philosophical question of how we can know if anything is real. Contemplating this question gives us the pleasure of feeling clever and feeling superior to the lesser minds who go around blissfully believing everything they see. It is also relevant to the story at the point it is given (as best I remember) because Neo needs to know the nature of the thing he is fighting.