Writing

I just generally describe it as well as I can then accept that every reader will envision it completely differently.

That's the best thing about reading, in my opinion.

Hobbyist/wannabe writer, but here are my two cents. I actively try not to convey my mental picture exactly the way I picture it. The only exception is for first impressions of people, like a specific fashion style or characteristic that says something about the person. If I find that something else depends on a very specific mental picture, then I’ll try to rewrite it so that it doesn’t.

It's hard to say. You definitely don't want to be too specific, but you do want to hit the high notes. What parts of the movement are important later, even if "later" is only a sentence or two away?

Could you describe the movements relative to the rest of the scene you're setting?

"Part of the mechanism moved along the west wall, as though guided by (mechanical/spooky action related to plot). It stopped, then rotated towards (character) but came to rest on its corner before completing even a quarter rotation to the east."

Just write to the best of your ability. Writing is a written medium, not a visual one, so unlike a film, TV series, or game, you can't guarantee that a reader will always see exactly the same thing as you. The imagination is a funny thing, which runs a full spectrum from aphantasia (an inability to imagine at all) to fully photorealistic "movie in their head" mental images. Nothing you write will ever allow those on one end of that spectrum to see what you're seeing, and writing all the details to make the other's mental images 100% accurate will bog down your writing with a lot of detail that just isn't relevant to the story. Also, the more specific you make your descriptions in an attempt to beam what you're seeing into the readers' heads, the denser your prose becomes, which risks losing readers with average reading comprehension skills.

If there's a specific aspect of the movement that's very important, then describe it as best as you can. Readers don't need, and won't remember, irrelevant details. If they do picture something incorrectly and something you write later contradicts it, then they'll edit their mental image and/or go back to check what you wrote earlier. I do this all the time when reading if it turns out I've pictured something incorrectly, and I'd say I have a pretty average visual imagination: I can picture things fine, but I don't have a "mental movie" that creates photorealistic detail. You could describe something in massive detail, and my mind will generate an Impressionist painting, not modern CGI.

Outside of combat, I have very little that needs to be described in that much detail. I do have one specific mechanism in my Living Dungeon story that I felt needed that level of detail, and the description went through a few edits to fine-tune it to communicate the idea accurately but keep the writing clean enough to not be jarring to the reader. I am fortunate enough to be able to get some feedback to help me in that process. I felt that this mechanism needed to be accurate because it is relevant at several points in the story.

The less relevant it is to story beats, the less I am worried about specific details. I have a second mechanism involving large colored, glowing crystals in a ceiling. I described the basic layout and the colors, because they were relevant to events in that room, I did not describe the shape of the crystals, because it doesn't matter.

In combat, how much detail I go into depends on the fight. Less important/low-stakes fights get more of an overview. A duel or spar with setup and some emotional charge? I am getting down to blow-by-blow, especially if I want to show the progression of a character's skill or power, and when getting that detailed I often need to specify which hand a person is using to do what, so that I can line up the action properly.

A right-handed person has a shield in her left hand, and therefore deflects a creature's charge to her left, and is thus spinning left to slash her axe across the back of its leg. I could have described it as her spinning counter-clockwise, which would be more technically accurate but would pull the reader out of the story more to think about. 'spinning left' suffices.

So, I would mostly just try to be sure that it needs to be described in detail.