When the water temps drop the metabolic rate lowers. At a certain point reduced blood flow to the brain induces coma, then the organs shut down and the the heart stops. There is no over stimulation to the nerves.(what we call pain)
I put the fish in a small bag of water in the freezer before I leave the house for the day. When I return the frozen bag & the deceased go out to the trash and life goes on.
This follows along the lines of the "shock" method and is much easier to stomach. This is a gradual temperature decrease, so the metabolism slows and so does the fish, but it is not being suffocated at the same time. What bothers me is fish being removed from the water into an uncomfortable environment and then being exposed to cold air which surely hurts as they try to breathe it. I've had frost bite before and it hurt like hell as it started to set in, stopped hurting, and then the thawing hurt like hell again. So I think that the "fish out of water, into the freezer" method is somewhat barbaric.