I consider it the final kindness I can bestow on the birds I raise with care for their meat, to help them Get To The Other Side Of The Road quickly, calmly, humanely. Make sure your knife is sharp, you have it placed right on the bare spot right behind their jaw, and that your fingers are out of the way. I press in deep & hard and push forward fast to get right to where the blood comes out in a gush. They cross over to the other side with that initial loss of so much blood, but I also cut the other side of the neck right after. Don't be alarmed by the delayed residual reflex of them moving and flapping, it's just nerve reflexes and not the bird trying to fly away. I stand there to hold the head and keep the blood from spattering all over but that's not necessary.
When you cut the neck but keep the head & spine intact the heart continues to reflexively beat and helps pump more of the blood out. This is also called a kosher kill and seems to make the most sense to me. But everyone eventually finds the methods that work the best for them.