To add to what Nev says, TLJ makes it clear that whatever technology they’re using to follow the Resistance is new.
When the First Order shows up to their rendezvous point, Leia comes to the conclusion that they tracked them through lightspeed. Finn says, “That’s impossible.” So clearly this must be state-of-art stuff.
Rose mentions that it is a new tech, but the principle must be the same as “active tracker”, which is how they figure out a way to potential disable it. I’m not really sure what active tracking means, though, or why only the lead ship would use it. The wookipedia article doesn’t clarify.
It does mention some of the “science” behind the tech though: Part of the technology used in the hyperspace tracker was a complex static hyperspace field generator, which enveloped arrays of databanks and computers in a localized hyperspace field that accelerated their calculation speeds to unimaginable rates.
That description does remind me of an idea I had about the reason the Holdo maneuver was so effective was because a part of the Supremecy was actually inside hyperspace in order for the tech to work, sort of like a periscope on a submarine. And hyperspace collisions can be extremely catastrophic, which is what makes this particular situation extraordinary.
I personally don’t think an explanation like that is necessary in the film. I also think the Raddus’ advanced shields is a good answer as well. You know, just something to quell the people that had questions like “why don’t they do this more often”. If it something made this situation unique, it would help explain that.
The “science” explanation also makes me think that hyperspace tracking basically utilized a hyper-quantum computer to either do two things, 1) slice into a ship’s navi-computer to determine where their next destination is, or 2) take the raw data of the ship’s trajectory to determine their destination out of thousands of variables with very high accuracy. A third possibility is that if each ship gives off their own hyperspace footprint or data shadow, that if the tracker can analyze that signature, it can then look into hyperspace and be able to follow it like bread crumbs. Maybe it is similar to the explanation they gave about the damaged Tantive IV leaking something, which allowed Vader to know where they went.
Whatever the mumbo jumbo answer is, it has to function in a way where the First Order can pursue them only moments after their target jumps to hyperspace, since Finn seems to imply that if they were to jump again that the First Order would arrive in “30 seconds”, making the effort pointless.
For my earlier suggestion about the loading bar for the hyperspace tracker, the explanation would have to fit with the idea that they would need to analyze something about their ship, either the navi-computer data itself, or study a ship’s “digital footprint” or whatever, and that process takes a few minutes. It also does give the technology a weakness that could be exploited in the future by the Resistance (if the First Order shows up, retreat before they can lock on to your ship’s signature, or everyone jump in different directions, etc).