I agree. Just give them a hard flick on the top of the head, and it should completely . . . for lack of less gross terms, crush their skull and scramble their brains. If you're un-sure of yourself flick them twice, it's still faster than any other method even if you have to flick them twice.

They will twitch, it's just nerves misfiring.
They may also bleed out of their nose, so if that freaks you out, wrap them up in a paper towel and them flick them.