I do this as well. I hate the sight of my birds spasming after we break their necks, but the stronger spasms help me to know that it was quick and that they likely felt very little if anything. I don't like processing, but it helps to process it like "this was a clean one, I know how to do this now" instead of taking in how the body is currently behaving after the killing blowI use the broomstick method and I found it easier once I realized that the really STRONG spasms came from the birds where I'd gotten the fastest, cleanest, easiest kills.
If you think of it as a signal that the kill was fast and merciful it helps.