(From TJ in the Raid Elite Discord)
So basically, the smart people on the internet reverse engineered (or just data mined more likely) the "results" of an active den. From there, there's a 3rd party program you use to spit out those results for you.
So if you go in and catch something from a den, then enter the stats into the program, they will give you thousands of results of the "next" pokemon to come out. It's stats, nature, ability, even shiny or not. Because as soon as a den is active, all that shit is generated.
Then, the first Mon you caught is considered your "seed", or Day 0. When you find the one you want (let's say 6 IV, or shiny, or whatever) you know how many days you need to skip forward. Not sure if you're familiar with the day skip trick, but that's a lesson for another time.
Once you skip enough days (if you're lucky, just a few hundred. Sometimes...a few thousand) and go back into the den, it will have the characteristics you want!