I was wondering what kind of advice do you have to avoid info dumps in fan fiction? Granted this can usually be avoided in fan fiction by sticking to what is known. But when you work with au's or other canon divergence, how do you handle giving the info of what is different without boring your reader with a huge info dump?
Drip-feed; only mention where you're differing from convention where it matters, when it matters. Sorry I don't do direct fan-fiction not in my writing nor my reading but let me try and elaborate without specific examples of fan-fiction.
Terry Pratchett's Capre Jugulum comes to mind, the line "everyone who knows anything about ... knows" gets used on a number of occasions to outline the things that people think, about both vampires and witches, that just aren't so. In this way readers are led to an understanding of how the rules do and don't work in this particular world through a number of small incidents rather than it feeling like a book of rules about the powers of witches and vampires; which is kind of what the novel actually is.
Tell the audience only what they need to know to understand the piece in front of them and only tell them where you're breaking with canon when it makes a serious difference.
First, Ash's "drip-feed" is good advice. We are trying to avoid info-dumping, not information in general.
As for technique, my personal favorite is through the thoughts and memories of the POV character. This can take some engineering on the part of the author to produce scenes that force the information out.
Mark shook his head. "I won't do that, sir, the engine will explode."
Wen Li was confused. Surely Mark knew the engine wouldn't explode, that was the whole point of him installing a Markhan monitor in the first place, three years ago. What game was he playing?
"Wen?" the captain said. "Is that true?"
Mark did not even glance at her, he was cool and composed. Screw it. "Chief is absolutely right sir. I'd have said so myself."
And that means, the new captain doesn't trust Mark, and Mark doesn't trust him, and that boy better explain why she just risked her grade lying to a captain.
Give the (or a) POV character a reason to think on the information you want to convey, add a dash of conflict and the mission is painlessly accomplished.
Note that avoiding info-dumps is pretty much always much longer than just telling people. In this case, "The Markhan monitor keeps the fusion engine from exploding." But that is what it takes to weave the info into the story.
Edit: And besides that, trust in your reader's ability to infer things from relatively minor clues. We do that IRL all the time when we meet people, especially new people, and we are conscious of their dress, language, grooming, accent, etc. Not everything needs to be revealed explicitly.
"Mark! Haven't seen you in years!" He held his hand out to the girl. "Hi I'm Jeff, Mark and I came up in Boston together."
She shook his hand, Jeff noticed a wedding ring on her left hand as she shifted her purse.
"I'm Jen. Got to go, daycare." She looked at Mark. "See you at home. In fact, grab a pizza."
Jeff doesn't need anything else to realize Mark is married to Jen and they have at least one kid, and it isn't their honeymoon, and Mark knows what Jen wants on a pizza and from where. This is just a bit of personal information, but the same applies to world building. Don't treat your readers like third graders, or be so paranoid you have to spell things out for them. You can find ways to hint at it, and making inferences can be part of the fun of reading.
Introduce one or more characters who know less than the readers. It's not a bad idea to always have someone in every scene who knows less than the reader, or to have some characters who are in the dark about parts of the story and others who are in the dark about other parts.
If you really can't figure out how there could be someone who doesn't know something about the scene, then perhaps the reader doesn't need to know it either.
How did the original material introduce its setting, characters and mechanics to you? Exactly. Just do that, and chances are people who liked the way the original series did it will like the way your fanfiction does it as well. And if they didn't like the way it was done in the original series, then they probably won't be searching for its fanfictions, anyway.
One good piece of advice I got is that it is not info dumping if the information is necessary. For example, I wrote a lot of Animorphs fanfiction back in the say.
Every single book of the Animorphs series begins as of you never read the series before. So it's two and a half pages or more of the characters reminding you who they are, what they can do, and why they are fighting an alien race, etc. This is not only necessary information but it's in keeping in style with how the books are written.
Of I was writing a Pokemon fanfiction, I don't need to tell anyone what a Pokemon is, or what the pokeball does. I can write a scene where the trainer finds Nidoran and show the reader how everything works in this world.