Unless your existing “fast” technique already looks like this, with this much super obvious forearm involvement, I don’t recommend trying to copy this.
Just from what we see in TC, very few players actually look like this. I try not to take it personally, being a misfit! Kidding, I just mean, when players show us their “forearm” or “forearm-wrist” technique there is rarely as much obvious forearm as what I use in some of the techniques I know. They mostly just look like wrist players with a small amount of forearm motion.
If that’s you, then I recommend following the instructions in the Primer for wrist motion. The presence of a small amount of forearm doesn’t change the instructions for learning these techniques. Some amount of forearm is almost always present at various points, even in very “wristy” players like Andy Wood.
TLDR I recommend doing whatever is most similar to your current technique and not worrying about small variations in the motion. The big-picture form of how to hold the instrument, anchor, and move, are the most important steps to get any picking motion to work.