getting into the head of the character slows everything down: it seems to detract from the spike of surprise due to the sudden action in favor of a slower buildup of psychological tension. I do not need psychological tension in this passage. There is psychological tension in the paragraphs leading to it, and this passage should just burst it like a bubble.
If it's muscle memory for her, there isn't much going on in her head. I'd advise to not get into her head because she may not even consciously be taking these individual actions anymore.
I tried a description of the action. She quickly grabs the gun, and simultaneously hits the wrist; before the other can react she twists the barrel, and steps back... and it sounds choppy like a laundry list of what-to-dos advices from the youtube videos I have been watching.
This isn't particularly choppy if you break it up more. The approach isn't flawed, but the implementation needs polishing.
While quickly grabbing the gun with her left hand, she simultaneously strikes the gunman's wrist with her right hand. Having loosened his grip, she twists the gun and forces him to release it. By the time he realizes what has happened, [she] is standing two feet away from him and he's staring down the barrel of the gun he was holding two seconds earlier.
Still not perfect, but it flows better and is not a bad way to handle this scene.
Also, because this is such a rigid sequence of events that is hardwired into her brain, you can actually get away with the underlying tone that this is a well-practiced routine. A great example here is the Sherlock Holmes fight scene where Sherlock skims the bullet points of the sequence of actions he's going to take.
If this skill was drilled into her (e.g. military boot camp), then it stands to reason that she lists off the individual instructions as she does them (if she even thinks about it at all, because it may have become subconscious musle memory by now).
I also tried skipping it altogether. She is held at gunpoint in one sentence. She is grinning with the gun in her hand in the next. This would work if I had showed it once, but this being the first time happening, I need to show something.
You don't need to show it once. You simply need to showcase that she has the ability. For example:
- After the unelaborated gun switch, have her show off her military tattoo that clearly proves she must have the skill (e.g. Navy SEAL tattoo)
- Have someone witness it and talk to her about it after, at which point she elaborates that her (military/cop) father drilled this into her as part of teaching her self-defense.
- Have someone witness it and talk to her about it after, at which point she elaborates on her aptitude during police/miltary training.
You're right that you need some kind of proof that this skill isn't just lampshaded, but you don't necessarily need to do that by describing the actual movement.
I finally tried being more abstract with metaphors and similitudes relying on the ideas of speed and force. Her hand hit like thunder, and she whirled the gun away from the other's fingers. It was a wet rock slipping under the greater will of the sea. And while it may be appropriate for martial art fiction, it is breaking the style and setting of my thriller.
I'd stay away from this because it's not the focus of the story. This isn't the story of someone with exceptional upper arm strength, or someone who is mustering more strength than she's ever had before.
Thrillers tend to go for bleak realism over the more flashy description you're using here. Your description sounds more like a superhero action movie.
Question: I imagine that this issue is more general to quick combat scenes, which resolve in less than a few seconds. How to get a pacing that surprises the reader and renders the swiftness of the action?
More often than not, surprise is used when it surprises the characters too. In that case, narrating things from their point of view inherently entails experiencing the surprise.
E.g. when a swordsman is so skilled that they cut the opponent before they even realize, write about how the opponent stood there and then noticed that he had been cut. The reader automatically has to reverse engineer the situation (there's a cut wound, therefore he must have managed to wound his opponent) just like the surprised opponent has to.
Or as a second example, when talking about a surprise assassination, first reveal it by the target feeling a sharp pain in their back before you actually talk about the assassin standing behind the target's back.
You can take a lot of inspiration here from movies too. As long as you narrate it in the same order that the camera shots take place, the same surprise will happen.