I did this once early on in my chicken keeping based on some really bad advice, will NEVER,EVER do it that way again. Slow, dramatic, traumatic, it was horrible. I sobbed afterward, what was intended to be a mercy felt monstrous. Decapitation, cervical dislocation, killing cone, is much, much kinder. The killing cone is somewhat tranquilizing while they are upside down. Despite being hands on, rather than hands off, it's much, MUCH less traumatic for everyone, especially the chicken. I still find it upsetting when I have to do it, but knowing it's quickly over and almost painless makes it bearable. Honestly, exhaust fumes is NOT the way to do it.