... It's the "almost" in "almost before they feel the pain" that disturbes me...
When you make that first deep cut, there is a whole lot of blood loss in those first few seconds. That amount of blood loss puts the bird On The Other Side, the blood pressure drops drastically, there is no more conscious brain function. That's why you use a veryveryvery sharp knife, so that it cuts deep without pulling or dragging across the skin & muscles, so that there isn't much pain to feel. The same way you can cut yourself with a razor or very sharp knife and may not feel it until you notice that you're bleeding.
I can understand how some folks are concerned that their chickens would feel even a few seconds of pain before they die, and try to devise methods to put them to sleep before dispatching them. But I wonder if all that kind of handling and gassing, while reducing the feelings of pain, may also increase the feelings of stress or anxiety.
I feel assured that I am providing a compassionate and humane end for my meat birds by calmly placing them in a holding cage the night before (no running around chasing and catching, just a simple lifting from the roost), by gently picking them up out of that cage when it's their turn to Go (again, no chasing around the cage, just a reach & grab), by stroking them and speaking to them soothingly before putting them into the cone, giving them a moment to grow more calm inverted in the cone, still speaking to them soothingly, and then making that deep decisive cut that makes them Cross The Road in an instant.
I was looking for a good video on YouTube that shows the Slice method, and how efficient & effective it can be. There sure are a whole lot of choices when you search for "chicken processing" and variations on that! Some of them are worth watching, others could cause you to eat tofu for the rest of your life. Here's a few I thought showed the Slice method done fairly well: