The lights going off is always ominous. Whether it be electric lights, candles, lanterns, or flaming torches, the sudden extinguishing of any of these never bodes well.
Firstly, you're now in the dark. Where dangerous creatures lurk and you can't move around without bumping into obstacles, tripping over items, or falling off cliffs. Secondly... what turned the lights off and how did they do it?
Answer these questions in the scariest way possible, and you have an instant pulse-quickening encounter.
Commentary by memnarch (who has not seen the movie)
Suuuuuure it wasn't you, Rey. It's only an experimental hyperdrive modulator that you have no idea how it actually works, what could possibly be your fault here? The star destroyer it was salvaged from probably didn't crash because of pilot error. Maybe you didn't actually fix the hyperdrive? The Falcon needed unspecified hyperdrive repairs before, but Chewbacca needed the power off to do so. As far as I can tell, all the lights stayed on until you plugged the modulator in. Maybe Chewie knew a little bit more about the risks involved with hyperdrives which could be a different skill than Repair.
New prediction; this modulator is some sort of time travel or time control device. If I told my past self any form of time control was ever going to happen in Star Wars, past me would have looked at current me funny and tried reconfirming that it was really us. Not any more! Time shenanigans are definitely possible in the Star Wars universe now and there's not really any other way to read the phrase "chronometric fluctuations" without a time something-or-other or coming up. If there's a relativistic reason for the vibration source, I will be shocked. Star Wars is science fiction is science pretty much only by genre as far as I know, so to have Star Trek level technobabble and anomalies is a big change, if true in the movie.
Commentary by Keybounce (who has not seen the movie)
Ok, this was a big laugh for me.
"Only light technobabble, so it shouldn't be too serious". Hardly. As soon as you hear "Tractor beam locked on to the ship", this isn't just technobabble - you're not going anywhere until you break the tractor. Hearing "chronometric fluctuations" implies some sort of time shenanigans, and that is always of interest. And Black Alpha flux tells us that this is relevant to past plot points - definitely not "light" or "not serious".
So what's likely to have happened?
Well, we know the Comic Irregulars have gotten to see all three of these movies. They know what happens in Episode IX, and probably have added a time travel subplot. We know that this is the point where this experimental hyperdrive was just installed and readied. And we know that the best concepts for real time machines will not permit them to go back in time before they were turned on.
So this experimental drive modulator is some sort of timey wimey wobbly hobbily device. And, there's a time loop subplot to try to do something to stop them from doing what they are about to do. But if they are trying to stop them from doing what they do, then they do what they are doing and they will do the thing they do.
So how can the future self stop the future past from bringing about the future that is trying to stop it? Ask Doc Brown and Marty. They managed after Biff's intervention.
How will the Comic Irregulars manage to fit something that crazy into a story that doesn't have it? Well, one way would be to have a strange intermission session that seems to have different people - perhaps our heroes in disguise (I mean, we've seen advanced genetic technology, so it's well within the established science of the setting) going to a strange, glitzy world to get ahold of a key document that will prevent this horrible timeloop.
This is so crazy, it just might work :-)
Rey: All right, the modulator is installed in the hyperdrive. Let’s get out of here.
GM: Suddenly the lights go off and emergency lighting comes on.
Rey: That wasn’t me!
GM: The ship shudders with an ominous time series of vibrations.
Rey: To the cockpit!
Rey: I scan the instruments.
GM: You read a high Black Alpha flux, odd chronometric fluctuations, and a tractor beam locked on to the ship.
Finn: That can’t be good.
Rey: It’s only light technobabble, so it shouldn’t be too serious.