To anneal brass it needs to get to 475 degrees F or there abouts. If the head gets annealed, the case is ruined, so the head area should not be heated to 475 degrees. In order to anneal the neck/shoulder without ruining the head, you must heat up the neck fast so that the heat does not migrate to the head. Brass is a good conductor of heat, so not only is the neck heated fast so that the heat does not go to the head, the head is heat sinked by setting in water or some other way. Hornady makes an annealing "kit". It has three different sized aluminum spinners and some heat sensitive paste. The aluminum spinners act like a heat shield to protect the head area. You put some paste on the shoulder and when it melts, the case is annealed and should be dropped into water to cool the case quickly. Some people set the cases into water and when the case mouth gets up to temperature the tip the case over into the water. After you use the paste a while you learn the color of the case and do not need the paste any more.
Brass gets work hardened over time, from squeezing the case down stretching it back out from firing, the redoing it. Depending on how much the brass is "worked" it will get brittle and split or fail sooner or later. Some people simply throw their brass away when it reaches this point, which can be 5 times being reloaded, 10 or 25, depending on how much sizing is taking place and case design. In the case of some wildcat cases, the case neck can split after as little as 1 firing. If cases are annealed, it can dramatically extend its life. If you put all the time into forming, then fire forming loads, you do not want to loose a case after 1 or 2 reloads. You can increase the life of regular cases by annealing and with the cost of new brass or factory loaded ammunition, so why not anneal?
BTW - I do anneal cases, but do not anneal before fire forming the 22K Hornet. I only neck size my 22K brass and shoot mild loads so it takes several sizings before they need annealed.
Good Shooting and Good Luck