I’ll second what Brooks said. Couple thoughts though…
If you’re euthanizing , cervical dislocation (broomstick method) seems like a preferred method.
If you’re trying to process the birds you’d probably be better off exsanguinating them. Kill cones work the best for this. Apparently turning them upside down puts them into “some kind of trance”? Maybe it’s the extra blood pressure to the brain, i don’t know. Between the two (cut throat or decapitation), I’ve seen processing videos that suggest actual decapitation might cause blood vessels to constrict and not allow as much blood to drain. So, maybe cut throat would be preferable?
All of this assumes that you could perform the act confidently and efficiently. None of these methods would be preferable if it’s going to be traumatic for you or the bird.