I can’t find if tvtropes has a page for this one. I’d be really surprised if they don’t.
Hero is a Fish out of Temporal Water but with an addition. He gets involved in a future war, and ‘invents’ tactics of a war that’s just been fought in his original time. No one remembers these in the future time, so they’re highly effective.
If you have an example of a work that contains that trope, you can check on that work’s page.
I have two examples: Armegaddon 2419 A. D. which is the original Buck Rogers book and Book of Ptath by A. E. van Vogt. Neither seems to have a specific page, although there is one for Buck Rogers (comic strip), of course. Couldn’t find it there, most likely because the trope probably didn’t occur in the comic strip.
The page for the Buck Rogers TV show names him as the Ur-Example for the trope of “Like A Duck Takes To Water”: getting the win with 20th-century dogfighting skills in a time when everyone else relies on shipboard computers, adapting football-field tactics for use against people who’ve never even heard of the sport, knowing exactly what’s going on when the bad guys come into possession of centuries-old weapons that our hero is a comparative expert on, and et cetera.
thanks for reminding me MEtv has buck rogers on tonight … I think ill make my aunt watch it again … I caught the tail end of last weeks episode and got all teary-eyed in squeezy cheese nostalgia as I was about 5 or 6 when it was originally on
With special guest star Jamie Lee Curtis!
This reminds me of the Isaac Asimov story “The Feeling of Power” which was published in 1958. Except it didn’t involve time travel; just a regular guy who re-invented a forgotten skill.
I could be wrong here, since it’s decades since I read it and I don’t remember any plot details, but I suspect that occurred in Twain’s A Connecticut Yankee in King Arthur’s Court.
That’s kind of the reverse: the protagonist goes back in time, and uses knowledge of things that hadn’t been invented yet (predicting an eclipse, the telegraph, etc.)
Actually, wouldn’t *Idiocracy *qualify? In the future, people were so stupid they’d forgotten how to grow crops. The time travelers from the past brought that knowledge back with them.
That’s not exactly it, but perhaps it’s as close as tvtropes has. Thanks.
King David’s Spaceship by Jerry Pournelle. A couple of the main characters introduce the phalanx and a few other “lost” military ideas to a culture paralyzed by a religious dictatorship.
I looked through TVtropes’s main future index We Will Not Use an Index in the Future - TV Tropes and didn’t see anything that looked likely. I did however find on the Atomic Rockets site their version:Everything Old is New Again ,which cites several examples throughout that site.
Find more examples and suggest a variant: Like a Duck in Temporal Water.
That’s another case like Connecticut Yankee. What I’m thinking of is the opposite. That is, the culture is so high tech they’ve forgotten the low-tech tactics of the past, whereas the hero remembers because they were used in the war he’d just fought in.
In my two examples, Rogers uses tactics from WWI (which may seem really strange, since our view is that that was a war with really bad tactics, in general) and the other book uses blitzkrieg tactics from WWII.
This happened in Star Trek IV - the Enterprise crew goes to the past and (IIRC) Scotty and McCoy go to a certain person in charge of a company for some special kind of plastic that could be used to make a whale tank in the Enterprise. Scotty designs it on the guy’s computer, and when McCoy objects that he’s revealing knowledge of the future, Scotty tells him that this is the guy who invented that type of plastic in the first place.
ISTR Scotty not being completely sure about that. 
“Transparent Aluminum”
And as I recall it, Scotty traded him the formula for transparent aluminum for the readily available plexiglas.
Not the future but parallel reality, but “Lord Kalvan of Otherwhen” by H. Beam Piper seems to fill the bill.
Yeah, I know I was a bit fuzzy on the details, but the scene seems to fit what the OP was talking about. If there’s a TV Trope for it, he might find it in the Star Trek IV page.