There've been a lot of answers already -- and I have read the entire thread -- but just wanted to share that I used a cone to harvest 7 meat birds last year. Put them in, head down, and quickly sliced the jugular, and let it bleed out. This is the method Joel Saletan uses (as well as Justin Rhodes and many other homesteaders whose channels I follow).
Joel states, in I think the same video which someone linked in this thread, that they lose consciousness when upside down, and the slice causes them to flatline within a second. It was hard for me to believe that, because each chicken appeared perfectly conscious and was struggling while upside down, then continued to struggle as it bled out. I have to hope he's correct, and you know, he raises thousands of birds and is widely respected as an expert.
It was really hard to do; Cornish Cross hens are a mellow, friendly breed, and when I picked them up for the final journey to the cone, they would bury their beak in my chest. I just wanted to experience it. I will likely do it again, but it does take shedding some of our city inhibitions and timidity and facing the cruel facts of Nature.