It's all insane alchemy. Just read the white papers over at annealing made perfect (AMP) -- just buying one of their machines is considered over-the-top by most hand-loaders, but they go on to do extensive hardness testing, microscopic inspection, and they use a transducer-equipped seating press (which they sell) to measure the consistency of neck tension/friction.
Obviously, they are going beyond just trying to extend brass life by preventing split necks (something I've never had happen with anything but range scrounge which I stopped using 5 years ago.) They are annealing to try to attain consistent neck tension -- or something like that, because there are arguments about whether it is spring tension of the brass, hardness, friction, and even whether wet tumbling and lubes affect the result and how.
Understandably, to get consistency, you want the brass cases to hold the bullet consistently from case to case so that a consistent amount of force is used to push it out. That's where the seating force meters come in. At first, there were dial meters like those from 21st Century. Now there are electronic transducers with computer graphs that show the force plotted across the bullet seating depth. The idea is that you would use cartridges that had similar plots to shoot a group.
Here's the thing: when you look at the actual results on target, it could all be snake-oil. I have not seen an obvious and distinct advantage from any annealing. I'm pretty sure bench-rest, PRS, F-class shooters are going to keep doing their voodoo, and I'm not about to prove them wrong by out-shooting them, but in their own tests with controls, there's no evidence it works.
FWIW, I have practiced annealing with a torch. If the torch is aimed correctly, I can hold the base of a 357 Magnum or PPC-sized cartridge in my bare hands and heat it while spinning it until the neck just begins to glow dull red in low light and drop it into a metal bowl before my fingers burn. That assures I'm not overheating and softening the head. Typically, I'll do this and measure the time in seconds. Then I will proceed to do all the cases of the same type based on the time estimate. For a single propane blow torch (Bernz-O-Matic), it's usually somewhere between 7 and 12 seconds. With that said, I would guess I am achieving stress relief, but I have not ever had one of my cases (annealed (or whatever you want to call it) or not) split. I'm not convinced it affects the end result whatsoever. It's relaxing though.
