I guess it's not discussed because it sems so obvious. Bonk the fish on the head and it stops wiggling. Most flyfishermen don't kill enough fish to carry a special tool for the task. Bleed it if that's what's required (very fast and humane). I have read all sorts of techniques: straight into the ice chest, apply liquor to the gills, stick a screw driver into it's brain, cut out it's heart, slit a major artery. What ever works for your harvest.
C&R, on the other hand is a more demanding skill. I suspect that we all could do better in that dept. May as well minimize the impact on the fish if you're going to let it go. If you're going to kill it, minimizing damage to the meat should be the priority.
One cast can change your day...maybe your life.